Why are they mad because she expressed her opinion?

She can say and feel whatever the hell she wants to.
Sure, she continues to do stuff involving fame, but that goes along with her career. However, there are people that still can do the music thing and go to events without having an enormous spotlight on them.
Katy is one of the many IT popgirls at the moment, so it's not fair to slam her when other musicians don't have to deal with as large of a spotlight.