Somewhat apropos of the post below, it's commonly stated that the only reason hot babes make it big in pop music is that the record producers pander to the lowest common denominator, namely raw sex appeal, and the devil if that means churning out shitty music. Like most poses that people strike to sound cool, we should take a good skeptical look at this. In particular, is the situation different in genres that don't pursue the almighty dollar?
I mention Hope Sandoval and Vega so often that you're probably sick of hearing about them, but they are two obvious cases of very pretty female songstresses who aren't very poppy. Since we've already talked about babes in classical music and lovely Gypsy chanteuses, let's take a deeper look into this genre. The folksy, lyrical trend of the '90s, and alternative / grunge music, was in reaction to the "excessive" '80s, so sex appeal should have played a minimal role. Even better, let's look at headliners of a concert organized along feminist, hippie-dippie, goddess worship, stick-it-to-the-Man lines. That, of course, is the all-female Lilith Fair.
So, for the two readers who are still with me, do a Google image search and tell me whether a single one of the performers from the inaugural Lility Fair isn't pretty. Remember to look for pictures when they were at their peak lookswise. Well, The Indigo Girls for sure, and Shawn Colvin may have looked plain. Aside from them, the rest of the 16 performers are pretty. Hell, some are even foxy: Sheryl Crow, mega-hottie Susanna Hoffs (from the Bangles), Leah Andreone, Emmylou Harris, and Fiona Apple.
That's 5 of 16 who would spin heads, 2 who are ugly or plain, and the rest somewhat pretty. Given the purpose of the concert, and given that it was aimed almost exclusively at females (males being dragged along by their grad student girlfriends), the "pandering" argument does not even leave the ground. I could think of many other examples of hot girls who rock out (Liz Phair), as I'm sure the readers can, so let's take that for granted and move on to why this pattern shows up.
We can rule out basic environmental factors, since it's not as if there's much malnutrition in the areas the above women come from, so being pretty and having a nice voice doesn't mainly show that you lucked out in your upbringing. Moreover, pretty and vocally gifted people are this way without much training -- to make it big, they may need coaching and a makeover, but their gifts are not due to coaching. Arguments that pretty girls are encouraged more to sing or play guitar are also unconvincing, since encouragement doesn't have lasting effects unless the person was predisposed to become good at music. How many parents push their kid to play the violin, and in how many cases do they play the violin for a living in adulthood, or even as a sustained hobbie?
That leaves genetic factors. There could be one or more genes that have effects in more than one part of the body, affecting both attractiveness and musical ability. Maybe, and it would be worth looking at what makes females look feminine, since females are better-looking and probably more skilled at singing than males, on average. (As with everything, males likely show more variance, accounting for their overrepresentation among eminent composers and opera singers.)
What seems more likely to me is cross-assortative mating between pretty girls and skilled musicians -- a well-attested phenomenon. You could test the "model and rockstar" hypothesis by seeing if the looks-singing correlation holds within families. That is, if mom is gorgeous and dad has a great voice, the kids will on average be above the population average for these traits by sampling their parents genes. However, if the two traits aren't deeply connected, it could turn out that the more alluring daughter isn't the one with the best voice. I think that's how it would turn out, but if it didn't, we'd be back to the previous paragraphs.
If this practice persisted for long enough, there could develop a micro-caste of attractive musical people, since the sons will be of the pretty boy rockstar type, and gifted babes would likely prefer them to ugly or plain rockstars. That still leaves aside why gorgeous girls might have preferred the musician when this practice began, but that's a separate topic from what accounts for the pretty songbird effect.