I don't have an issue with the lyric itself, the song overall is quite childish. As a songwriter though, I can totally see where she's coming from. She probably feels very embarrassed by the song overall not just that lyric, purely from a 'catty girl aggressive bullshit' perspective. She's not a kid anymore, it's an ugly representation of being a teen and Paramore have grown up a lot thematically since then. I can see why fans would be pissed though, it's like their biggest hit. But you can still listen to the record, and there's a loads of old live recordings. No one is really losing anything. Her song, her band, her choice.