Judging by your username alone, yeah, you are.   I'm not sexist, but I really dislike the majority of teenage girls
Are you making fun of teenage girls? What do you dislike about them?
Teenage girls are mostly annoying, so what if I make fun of them. It's a joke you idiot.
You can make fun of anyone you want, but you shouldn't name call someone you don't know. Generalizing all teenager girls as annoying is impossible and you could be more modest in your assumptions.