I think it's about Paul McCartney's unwillingness to put any real effort into his lyrics. He came up with a jaunty little melody, that he for some reason thought would be perfect for a comic song about a serial killer. Given that one of his songs had recently inspired the Manson Family to actually murder people for real, you'd have thought that some kind of basic good taste might have restrained him, but apparently not. A good example of late-Beatles self-indulgence. A shame, as the rest of Abbey Road is great.