The joke may not be that funny, but it's a joke. In what way is it transphobic? He isn't scared of Trans people, and he even used the correct pronoun when talking about Jenner. That's more respectful then most people are. As for offensive, there's this thing called freedom of speech. People are allowed to make offensive jokes about other people. People make offensive jokes about celebrities all the time. This joke was barely offensive, and worse things have been said about Jenner that she hasn't cared about.