Why are pretty girls so mean?

Not all pretty girls are mean.

Those that are, might be mean for the same reasons that anyone else might be mean.

However, there are some possibilities to consider:

Perhaps they think that bringing other people down will keep them on top. They do not realize that there are other ways to be popular, like being really nice so they'll have lots of friends, not just ignorant followers who are their "friends" just to cash in on their popularity.

They might also have a belief that they are better than others because of their beauty, and thus do not have to treat others well, but yet deserve to be treated well because they are better.

They might also just be used to having people tolerate their behavior, and have not seen consequences of such rude behavior.