Huh. I've just read this entire thread from beginning to end and have yet to see a song which makes me really scratch my head and wonder whether the writer was looney. Some songs are a given, as with Jim Morrison. Everyone knows what he was on. But even so, his poetry DOES make sense to me, if viewed from the perspectives of knowing at least SOMEthing about his own life. I'll admit that I don't like the Doors for the lyrics, but for the music, though I don't DISlike the lyrics. I'm certainly not one of those who thinks that Jim Morrison was a brilliant poet. A good one, but I think he trapped himself into that angsty state most of us go through in our late teens. If it weren't for his use of vocabulary, many of his lyrics would come off as very trite and cliche.
But that lyric about screaming butterflies, it really does make sense. The whole song is, of course, about death; about his death, perhaps, and about the death of the earth, of nature. A butterfly is so fragile, yet silent. Easy to hurt, yet never complaining (that we humans can hear). Like the earth. Perhaps it was a suggestion that we be more aware of the pain we inflict? At least, that is what I got out of it.
Same thing applies to Kurt Cobain as to Morrison. Folk songs are often put into perspective if one knows something about the era in which they were written, or the particular cause. A lot of novelty songs are just people being goofy, which is, I think, different from looney. Or maybe I'm just very good at finding my own meaning in things which I enjoy. Of course, some of us are just a little silly to start off with, too. Acting like we're on drugs comes natrually to us.

But, a lot of people wonder about these songs:
Ina Gada Da Vida and Fishheads (Fish heads, fish heads, roly poly fishheads)