First things first, Nirvana is singular. When you refer to 'em, it's "Nirvana were overrated". "Nirvana suck". Cobain wasn't Nirvana and vice versa.
Secondly, Nirvana were not and are not overrated. They're overhyped by kids who lament Kurt's death, even though they were 3 or 4 at the time, holding him in a godlike regard when alls he was was a guitar player for a rock band.
They cranked out a lot of great tunes and a signature sound, but they were far from revolutionary, music wise. In fact, they were better acoustic than live instrumentally, and Kurt, to his credit was a great, deep song writer.
Anycrap, everyone who ever covers the song, they do it because it's part of the clause when you sleep with Courtney Love, that you have to cover a Nirvana track, and give her a cut of your take from the door.