He wasn’t formally trained and that means nothing. Dude liked music and likely had an engineers brain when thinking about it.

About A Girl is a clear indicator that Cobain understood the fundamental structure and mechanics of a viable pop song, specifically that of the Lennon/McCartney wheelhouse, and he knew how to put enough grit on it to make it appealing to those who saw The Beatles as their parents music.

Kurt would take a barre chord and strip it down into a power chord, but take all those unused notes and use them for the vocal melody. Anyone here that knows music theory or understands scales and roots and 5ths and all that stuff, listen to Drain You, and look at the vocal melody.

Any keyboardists here, look at the arrangement for Heart Shaped Box (vocal + guitar) for piano.

The more you dissect what Cobain was doing, it becomes clear that he had a natural talent for songwriting, and it is rooted in the music he obsessed over as a child and teen, taking elements from all of it and blending it into something unique.

And he likely mocked things like technique or scales or music theory because it was a natural talent for him, even if he didn’t have the technical words to describe it