Depends on their age, I'd say

If the prettier a girl is, the more people allow her to act like a jerk (say for example humiliating someone and bossing him around), and she gets away with it just because she's pretty, then it's a lot more likely she'll turn into a total jerk. If, on the other hand, more people would refuse to put up with it (say for example, leaving, and considering commenting on the jerk's behavior) no matter how pretty she is on the outside, the less she'll be inclined toward that sort of behavior in the future. On the other end of the spectrum, if someone is treated like garbage because they are ugly, they are very likely to become bitter and resentful, and who can blame them?

Since jerks are not born, they're made, the younger a girl is, the less correlation there is between her sppearance and her attitude.