I think you're overstating the uniqueness of Cobain's "brand" and underestimating the size of the anti-corporate movement in the popular culture of the 1990s. First, it wasnt really Cobain per se—Nirvana were huge for a few years, but Cobain really only became a household name in death. More importantly the rejection of corporate influence and "mainstream" art was the guiding ethos of that whole genre, and eventually of lots of other artists as well . "Smells Like Teen Spirit" may have been the spark, but you could see the influence of that movement for many years, across the punk revival, riot girrls, zine culture, independent film, "culture-jamming" efforts like Naomi Klein's No Logo, etc (to say nothing of the glam, "hair" bands who were instantly put out of work by the advent of grunge). Not for nothing did the anti-WTO protest take place in Seattle.
Not overstating his uniqueness in being anti-corporate, as much as his uniqueness in being both anti-corporate AND mainstream. And you might be right that his mainstream appeal came later, but I think the point still stands.
This reminds me a lot of the tension the I felt when I was first coming of age as a young adult in the late 00s. At that point, it wasn't grunge, it was hipsters. But I always felt like: How can hipsters be hip and counterculture and a movement that is big deal culturally at the same time? As soon as a "counterculture" becomes a mainstream movement, it's no longer a counterculture...
So which is it? Was Cobain a big deal in the mainstream, or was he countercultural? I don't think you can have it both ways.