As Halloween has become adult-oriented over the past 20 years, the "sexy Halloween costume" has become standard, and was satirized as early as 2004 in Mean Girls.
For the most part, the roles that the girls assume are not inherently repulsive, which would thwart their main goal of exuding sexiness at a Halloween party. Some roles are typically warm and feminine like the pet cat or nurse. Others are threatening and dangerous but not necessarily ugly, like the witch or devil -- both of which may deceive with outward attractiveness.
The sole possible exception is the sexy zombie, whose bloody / wounded / decaying body is inherently repulsive. Even there, most girls try to dial down the gore and play up the pallor, vacant stare, and so on, so as to be less disgusting. At the party, they do not move around lethargically and clumsily like corpses, and their normal-to-high energy level makes them seem actually alive rather than (un)dead.
Since Halloween is a holiday for the carnivalesque inversion of rules and roles, the trend is for girls to choose the threatening-but-not-ugly category, rather than the safe-and-familiar category. For one night, they don't have to project propriety, and they can take on the persona of someone they are prohibited from being during the rest of the year.
A similar logic plays out among the guys at the party: for one night, they are allowed to openly desire a member of some group that is otherwise forbidden for them to get horny over. The rest of the year, they must only openly desire nice respectable types, not a witch, devil, zombie, leopard, lusty schoolgirl, etc. -- all of which represent various sexually taboo groups (spirits, dead humans, animals, underage humans, and so on).
Even on Halloween, they are not violating these taboos outright, but only bending them. They're not lusting after an actual leopard, but a human girl wearing a leopard-print body suit. Not an actual high-schooler, but a 24 year-old in a plaid mini-skirt and white button-down shirt.
This is the social control function of a collective ritual like wearing costumes for Halloween. The carnivalesque inversion of roles maintains the status quo by allowing for some exciting -- if temporary -- dynamism, to counteract what would otherwise be an oppressively tedious static system. And the more rare the occasion, the more intense its expression -- which serves to satiate the participants with one great big indulgence, rather than leave them forever craving more if they were to only provide a lot of weak hits. The massive dose sends them into a refractory phase, like a hangover, and ensures that they won't be bending the rules for a long while afterward.
During the rise of Halloween-for-adults, the society has become increasingly polarized by political and cultural membership. That has given the holiday a new source of dangerous, threatening, forbidden types to dress up as (for girls), and to lust after (for guys) -- those who belong to a different political party or a different sub-culture. Call this the "sexy enemy" category of costumes. Not a personal enemy who you, the individual, has beef with -- but who you, as a member of Team A, are opposed to because they belong to the rival Team B. Someone from a group who your own group would never allow you to marry or have children with.
The earliest example is the sexy nun, which I can only date back to 2005 (see here). That sacrilegious trope has existed for far longer, I mean its form as a Halloween costume. That came in the wake of the Catholic Church pedophile scandal of the early 2000s, but has remained common through today, as part of the church-going vs. atheist culture war. As with the other examples, nuns are not inherently ugly, and their roles run the gamut from nurse to teacher to disciplinarian. But within the groups where the costume is worn, their role is "evil religious extremist".
The sexy lady cop can sometimes take on an Us vs. Them character. Generally the Halloween costume party fanatics are on the left half of the political spectrum, where cops are a political enemy group from the right half. That is compounded by the racial divide if the lady cop is white and the male spectator is black or Latino. It's hard to think of a group that black men bitterly hate more than "white cops" -- but if it's a white-woman cop wearing booty shorts and fishnets, then suddenly it's socially permissible among their group to back the blue (for one night, anyway.)
More clearly political is the case of right-wing talking head Tomi Lahren dressing up as celebrity leftist AOC:
Here, she's not dressing up as a generic "sexy leftist" -- perhaps wearing a Che Guevara shirt and dying her hair blue for the night -- but specifically AOC. This means that she and her audience believe that AOC is hot enough to serve as a sexy Halloween costume. The red-blooded red-staters in her audience could never be allowed to marry or have kids with AOC, but they still want to "fuck the liberalism right out of her". That too would violate sexual taboos, though -- you're not supposed to even sleep with an outgroup member.
However, if it's not the true AOC -- but a right-wing babe cosplaying as her -- then it's socially permissible among them to lust after the Commie from Queens. It's bending the norms rather than breaking them. And for the rest of the year, these left vs. right boundaries become stronger after being bent and tested on Halloween night. As in the lady cop example, this case has compounded thrills due to the racial factor, with a Nordic right-winger cosplaying as a left-wing Latina.
There was a similar case in the late 2000s and early 2010s, where left-wing guys lusted after Republican VP nominee Sarah Palin, but could only express this in a socially permissible way if it were actually a left-wing woman cosplaying as Palin (comedian Tina Fey or pornstar Lisa Ann). I wasn't at parties with the middle-aged target audience, but Google Images turns up plenty of results for "Sarah Palin Halloween costume".
Notwithstanding these left vs. right examples, polarization is a fractal phenomenon. It's not just the highest-level group, like the nation, that is polarized, but every level on down. The left is highly fractured within itself, as is the right. And each fragment of the left (or right) is itself fragmented, and so on down the line.
So by now it's not just the left dressing up as the right, but one camp within the left dressing up as another left camp. For example, here is Anna Khachiyan's costume from last year. She's dressing up as a basic Becky from the East Coast suburbs who's a regular at Starbucks, has a marketing consultant career, and loved Hillary Clinton in 2016. In other words, a "fellow" member of the left half of the spectrum, but still separated by a sub-cultural divide from the Bohemian olive-skinned ethnic from the urban core who's a regular at Sweet Green, has a niche podcast career, and loved Bernie in 2016. Her audience is from the same in-group, and instantly recognizes which out-group she is cosplaying as.
Choosing the Starbucks Becky reveals that the guys and girls of her sub-culture believe that this out-group is attractive, even if they would distance themselves from them by saying "conventionally attractive". To them, Starbucks Becky's are so attractive that they can serve as a sexy Halloween costume. Anna's fellow urbanite arts majors are prohibited from marrying or having children with someone whose favorite musician is Taylor Swift, and yet they can't help but lust after their long luscious waves, thigh-high boots, and intimacy-enabling lack of irony poisoning. However, if it's not a true Starbucks Becky -- but an art-hoe podcaster cosplaying as one -- then it's socially permissible among their group to openly lust after the type. (For one night, anyway, then it's back to the sub-culture war, whose faultline has been clarified by this boundary-bending exercise.)
Finally there was the case of strengthening the boundary between two factions within the Bernie left, the stylistically radical neoliberals vs. the anti-woke left. Although technically not for a party since she was only hanging out with herself on Halloween, here is a radlib girl cosplaying as the anti-woke left princess, Aimee Terese. As with Tomi Lahren dressing up as AOC, this girl did not wear generic signifiers to look like a generic "sexy anti-woke leftist," but specifically modeled herself after Aimee's iconic Twitter avatar. Both she and her audience feel that Aimee is unambiguously hot enough to serve as a sexy Halloween costume.
Normally, the radlib soyboys would never be permitted to openly thirst after a boo-hiss class-reductionist and anti-feminist, as much as they might want her. But if it's not actually Aimee, only a radlib thot cosplaying as her, then it's off to the horndog races. For one night, anyway, and then the radlib vs. anti-woke boundary gets stronger for the rest of the year after it's been bent and tested.
I'll add other examples in the comments if I think of them. But that covers the phenomenon from the highest to the lowest scale of group polarization.