Sonic Youth had already been playing for 15 years when Nevermind came out. I saw them a bunch of times in the early 90s and yeah the crowds had no idea who they were because they were opening for Nirvana and Smashing Pumpkins despite having laid the ground work for those (in my opinion) lesser bands. Sonic Youth were too punk for the crowds showing up for the more commercialized sound that Nirvana ironically offered them.