How has fun. always been a mainstream pop band? One of the first songs they put out was an exclusive to this site and they used to play tiny shows and support bands like Jack's Mannequin and Panic! at the Disco. Aim and Ignite is hardly mainstream pop.
Same goes for Paramore or FOB. Do you think they're not mainstream?
You can't sell a million records, win two Grammys, and have three songs on the radio and not be mainstream