> So which is it? Was Cobain a big deal in the mainstream, or was he countercultural? I don't think you can have it both ways.

Both, and it's referenced in the chorus of the song In Bloom. They were becoming popular in the mainstream and attracted an audience who were the antithesis of what they believed in:

"He's the one who likes / All our pretty songs / And he likes to sing along / And he likes to shoot his gun / But he knows not what it means / Knows not what it means"

see also: people who sing along to their favorite patriotic song Born In The USA.