A common thing w/ Nirvana songs is that Kurt plays power chords, and then has the thirds (+ other tones) in the vocal melody.
But also, it was just a (counter-) cultural thing to feign lack of music theory knowledge or practice at the time. Quite a destructive one, I might add.
A nice video about it: https://www.youtube.com/watch?v=pWY4YYmSTWg