The post below on the disappearance of midgets from pop culture walked through the basic links from rising violence rates to a greater interest in the unusual. The other side of the no-more-midgets pattern is of course the disappearance of giants. Chewbacca, Jaws (the James Bond character), the Predator, Andre the Giant's character in The Princess Bride, Sloth from The Goonies, Bigfoot (where did he run off to anyway?), the giant in Twin Peaks ... we haven't seen much of a fascination with freakishly large people during the past 20 years -- CGI, etc., do not count because only a real-life human that big is a cause for wonder.
What about attractiveness? Here too you don't see the same attention to the farther-away-from-average folks. At the far ugly end, there was Screech from Saved by the Bell and Martha Dumptruck from Heathers. And at least until she starts to get made over, Sissy Spacek in Carrie is one of the few characters whose mere sight turned my stomach during a movie. She's not fat, doesn't have a horribly asymmetrical face, but there's something about her skin that just gave me the willies. The one who takes the cake, though, is that ranting crone from The Princess Bride. ("Rubbish, filth, slime, muck -- BOO! BOO! BOOOO!")
These are characters whose physical repulsiveness is necessary for the role, not who just happened to be played by ugly people. Aside from that girl in Precious, no one seems interested in studying what life's like at that extreme. It's not a real exception since it was never very popular, but the indie Welcome to the Dollhouse from 1995 also had an ugly girl in the lead role of awkward teenage misfit Dawn Wiener, unlike the cute chick who played Juno.
At the opposite end, the drop-dead gorgeous babes have all gone extinct as culturally visible types. Here is a somewhat recent Joel Stein article on the fall of the supermodel from the early-mid '90s through today. Women chosen only for their beauty no longer appear in ad campaigns, on the covers of magazines, and all the other places that people used to assume models would dominate forever. They've been replaced by actresses, TV show hosts, singers, etc. Even their most attractive members won't be the best-looking because they must also be able to act or sing decently, whereas models (used loosely) are free of that constraint, although they do have to be able to express a range of emotions.
Just like the characters whose ugliness is integral to their role, others require a mega-babe for the part. In both Vacation and Christmas Vacation, how are they supposed to tempt a gung-ho family man away from his wife, who is already attractive, unless they introduce a woman so stunning that we understand what made him give in? He's so committed to his wife that nothing less than a rock video vixen type could make him seriously consider stepping out. A woman like Cameron Diaz, Katy Perry, or Megan Fox does not have looks overpowering enough to fry the rational circuits of Clark Griswold's brain when she asks, "Well, are you gonna go for it?" ("This is crazy, this is crazy....")
In the past 15 to 20 years, we have gone back to the previous era of falling crime, the mid-'30s through the late '50s, when the popular sex symbols had already made a name for themselves in acting, singing, or something else. Bettie Page was an exception; most of those pin-ups were of established actresses, including the most iconic pin-up of Betty Grable.
Similarly, the last peak period of model visibility, the '60s through the '80s, was like the earlier era of rising crime, circa 1900 to 1933. It's true that the sex symbols of the Jazz Age like Clara Bow and Louise Brooks were actresses, but they didn't really act, being silent film stars. I'm sure that required more beauty-irrelevant skills than a model's job, but they were still seen and not heard. They were not known primarily for their competent or excellent acting skills -- meaning those that involve verbal communication -- and incidentally for their good looks. If she was a knockout and could express a range of emotions, that's what counted for her to become a sex symbol or style icon, not unlike the requirements for a supermodel.
And it's not as though the Jazz Age wanted for a supply of real actresses and singers from whom to draw sex symbols. Mass-market popular music had taken off, and so had radio dramas and comedies, not to mention live acting. It's just that, when the goal is to produce a sex symbol, why require them to be skilled at acting, singing, etc.?
Hopefully by the next time the crime rate starts climbing again, the whole country won't be bloated with obesity, and we'll get to enjoy another Jean Shrimpton, Cheryl Tiegs, or Cindy Crawford.