I know this is 8 years old lol but I can answer a lot of these because In pretty similar in comparison to Kurt... So it was never about being "lazy" with the unwashed hair... it's just something that punks do because if you don't wash it you get all of the natural oils and it makes it look better, lay better, feel better... When you shampoo hair like his or mine (straight light brown hair with blonde and slight red in direct light) it comes out all fluffy and weird. I would bet that he also does what I do and use a towl roughly on your hair while wet and after dry too because it really helps it look/lay down better and gets any waves or anything out.

You're right that Kurt had light brown hair same as mine and there was an era of footage where he has it like when he jokes around with that tazer he had his natural hair there clearly.

I wish there were more posts like this with activity because I'm curious about stuff too. You should repost this now I think it's been long enough and not just here but there's other Nirvana subreddits too