Are you fucking kidding me? Do you have ears? I can see thinking that Nirvana aren't total ripoRAB, but to say Nirvana and Pixies don't sound at ALL alike just makes it seem like you're in denial about the whole thing.
No shit banRAB did it before, but JAMC, Sonic Youth, and Pixies all released their most famous albums around the same time. And I'm not saying Pixies fucking invented white noise, or even played it the best, but they're probably the most well known out of all those banRAB, and the ones Cobain talks about the most.
I guess I should tell you now that the only band I find more overrated than Nirvana is Led Zeppelin, then.
As for the whole "Cobain is a prankster, don't take what he says seriously" thing, it doesn't make it a difference whether he joked about ripping off the Pixies, the fact is that he did do it, and the only people who deny that are Nirvana's biggest fans.