Katy's music is vastly superior to Carly's
It's just because Carly is such a flop that nobody feels threatened by her. Also because of the fact that her music is unsuccessful, people feel like it must not be as generic and see her as more "indie"
If the tables were turned and Carly was being played on the radio 24/7 and was ending careers like Katy did then ATRL would have a very different opinion of her and her music. As much as people will deny it, she'd be getting dragged for being generic, radio-friendly, "childish" etc.