I think it would be hard for Trump to win. Even if Trump didn't think it was a joke, I imagine most reasonable people know that Maher was making a joke and not a serious contract.
They laughed at the comparison of Trump to an orangutan but they only applauded his offer to donate 5 million to trump's charity of choosing. Nobody laughed at that and Maher sounded very serious making that statement. They've had legal professionals give opinions going both ways on this case. Most think he won't win but some think he could so I'm not the only one.