You can say Nirvana is 90s alternative rock. Think about it this way, there's this grey area in the early to mid 90s where it makes sense for fans to like loud, dirty music in general. Dinosaur Jr., the Dambuilders, Sonic Youth, My Bloody Valentine, the Melvins, all of them (or at least, most of them) take root in the late eighties and early nineties. It's the environment surrounding them that makes them identifiable. That whole touring circuit. Grunge is a stupid term that encompassed one scene in that whole cluster of musical offshoots. Kurt went to Metallica shows dressed in a Vaselines shirt and hung out with Dinosaur Jr. and the Melvins. Says enough about his background.