There's a difference between doing a joke and being rude. If someone doesn't like the joke and tells you to stop, that doesn't mean you get to keep going. It may be April Fools' Day, but have some respect for the people you are joking around with.
Theming is different, the YouTuber likely is not going to care if you use their likeness and don't actually claim to be them. If they do actually decide to check the Mario Boards and come across SGoW, then that's a different matter ^^;
But copying someone's theme without consent from the user, like what BBQ Turtle and I did or the LeftyGreenBabyLuigiOnFire twins did, and they tell you they don't enjoy it, then you should stop the joke.