Also, making fun of people's appearances is one of the most fundamental and universally appealing kinds of jokes

Yes but "you're fat" isn't making fun of appearance. It's just a straight insult without humor. "What's your blood type? Gravy?" would be both an insult and a joke. It's splitting hairs, but I can see where he's coming from on that one.

But he's wrong anyway because Trump has told jokes during speeches. "Last time I put up that chart, I never really got to look at it," was a great line.