How do you know that the "ugly" people are doing it out of jealousy?
I've never seen this happen in "real life", only with celebrities. I'm going to assume its the same kind of situation, here. People will call a celebrity who they don't personally care for "ugly", because they're so sick of hearing about how hot/beautiful they are all the time.
There's only so many times you can hear someone rave about how hot Megan Fox (or anyone you personally don't find all that attractive) is before you snap and say that she's ugly as hell. Is she really ugly? Of course not. But, people have this reaction to rebel things that are popular. It's because other people are constantly praising it that you feel some sort of "balance" by knocking it down a peg. This happens with people, movies, anything really.