I thought this way for all the years between when they became a household name until maybe some time last year.
I still don't really like their genre or their sound, or much of their music, but I feel that I actually get them now...and I get why they became so popular for their fans and for what was going on with music, especially popular music, at that time.
For me, I never could appreciate them (like their sound or their songs) because I never shared the angst that their fans--even at the same age--are generally assumed to share in order to appreciate Nirvana and Cobain. I just thought of them as loud, mumbly, and complainy. In a way, I still do, but I guess I get it. That actually was very serious and meant a lot to a significant audience of what became popular music from that era, so it has very real relevance regardless of whether you like it or not, and regardless of whether it should be considered over or underrated. At the same time, I much preferred Pearl Jam (I preferred their sound and their songs, even though it is pretty much the same thing in the end).
As far as bands that pretty much defined their genre and their generation of fans, with a heavily-identified front man, and that seem to generate rather polarized opinions, I can compare Nirvana to the Doors. And my over-underrating, appreciating of the two, is frankly quite different.
The Doors:
--generally my type of sound: I love the blues and Rock and Roll. I love hippy bullshit and anything with a solid piano. Hammond B3 a plus. I generally love all psychedelic tunes, especially with well-crafted, story-driven tunes.
--Front man has a great voice for R&R and the Blues.
---I vacillate between liking them and labeling them overrated. Here is why:
I love their music. It's catchy and even poppy in a way. ...I hate Jim Morrison. I think he has a great voice and his lyrics are generally OK, but it is all bullshit. He's a terrible writer and certainly not "a poet" as people want to think. He's a phony. The band is great overall, and doesn't suffer because of Morrison.
Nirvana:
--Definitely not my type of sound: loud, messy, heavily distorted bullshit that generally supplants lack of skill with some notion of "art." Fuck that. I can appreciate to a certain degree the notion that an orchestra of terrible noises is its own art and not some mere cacophony--I get that--but I don't want to pay to listen to that shit for a 2 or 3 hour show.
--Cobain has a terrible voice. He strikes me as a spoiled suburban twat that primarily complains about problems that exist only because he chooses to have them--suburban angst. ...but he actually isn't phony. These problems are real to him and his lyrics are exactly what his life is like which, not incidentally, really was quite fucked up. Much of which were his own poor choices, but he was raised in a rather shitty household with shitty people around him. Cobain is honest and in no way a phony, and Nirvana is as honest a ban can be with their sound and their music. So....I get it.
I like the Doors more because I like their sound, but they are a phony band compared to Nirvana, who is a very honest band with a shitty sound and shitty music to my palate.
Neither are overrated.
There.