It's funny because it's true:
In 2004, Mitt Romney made fun of John Kerry for being rich
Now, there's a senator from my state that you may have heard that wants to get elected president. I don't know why he wants to do that, because of course if he won, he'd have to move into a smaller house.
By the way, I don't which was worse: Mitt Romney's delivery, or the joke itself. I mean, if he was going to blow his ability to claim righteous indignation when the tables are turned, don't you think he at least would have liked to tell a funny joke? But on the upside (from Romneyland's perspective), at least Mitt didn't make a joke about a car elevator or home lobbyist or anything crazy like that.