Is this girl really mean and rude or is it just my imagination?

Not all pretty girls are mean and I'll bet you're not hideous. She's happens to be pretty because it would be near impossible to gain that kind of popularity and not be pretty. Most likely she is just jealous of how smart or pretty you are. I know it sounds cliche but it is so true. Almost everyone makes fun of someone else at some point in their life because they want something someone else has.