That is a very interesting explanation of why glass jaws seem to be more prevalent in MMA. Very plausible.
Actually, I think it's because the UFC, MMA's equivalent of HBO Championship PPV boxing, consistently recruit and show fighters with less than ten professional fights and also pay plenty of cans to come catch a beating from their TUF "prospects". So, in effect, you see Glass Jawed Jokes on PPV that are equivalent to the boxers you'd catch at Dirty Dick's Fight Night at a downtown strip club.