It's not understanding that can get condescending, but sympathy (with the same root in the word pathetic). When people start expressing sympathy, it's most often in response to what they imagine your life to be like, rather than who you really are, or what you're really going through.
And that's what's so tricky about writing a song on the subject: A song is meant to recall memories, and draw forth strong emotion. But so many cultural memories and attitudes around disability are distorted that calling them up might actually cloud your message, rather than make it clear.
Actually, I think the best song "about" disability wasn't written with disability in mind at all (I don't think): Roger Miller's You Can't Rollerskate in a Buffalo Herd.