I say both. However, saying “The joke is on you” is more about a specific thing/topic that you’re talking about. For example: You shouldn’t stay in a relationship if you’re being cheated on. If you decide to stay then, the joke is on you. “Joke is on you” is more general and can be about a conversation being had about multiple things. For example: I don’t think they’re your real friends. You should be more careful about what you tell them. Also, you should stop being too nice to people who don’t appreciate you. Just my advice, but if you continue to allow people to mistreat you, then, joke is on you. Hope this helps. 😊