Nirvana was only a part of a group of imerging bands from Seattle. Similar to the British Invasion (which spawed domestic imitators) there was the Seattle-sound music explosion of the early 1990's. Nirvana was the most POPULAR out of that group but that does make them an influence. Pearl Jam, Green River, Soundgarden, Alice In Chains, Mother Love Bone, Temple of the Dog, Stone Temple Pilots, etc. Of any in that group I think Nirvana wasn't the biggest influencial band out of that group. 2 albums and only a few years of performances before he offed himself.
Personally I can see influences more from Soundgarden and Alice in Chains than anything remotely from Nirvana. During the early 1990's, Mother Love Bone had A LOT more to do with the Seattle sound than Nirvana ever did.