There are several Weird Al Songs that are commentaries on the artist being parodied, most notably “Smells Like Nirvana” which is a commentary on Kurt Cobain’s unintelligible lyrics. Fat is a social commentary on the fact that Michael Jackson was, in fact, quite skinny.
Buy it: Original 2020 Made Me Snap Shirt
My guess is he was happy to sell it off and let go of the burden of living up to impossible expectations. And then, super classy of him to donate all of that. He’s already stupid rich and has everyone judging him. Smart and respectful.