Because of the way this is said I think it is probably a sarcastic comment. I don't think I've ever heard someone say this in a non-sarcastic way. They're implying that they aren't actually "a ball of fun". I expect they mean that the person is being a bit of a "downer", a bit boring, or a bit a bit depressing. For example: A: Everything is rubbish and the world is terrible. B: Well, aren't you just a big ball of fun. It could be used non-sarcastically but the "well, aren't you" part is a big give away that it is sarcastic. A similar response would be: - Well, you're just a joy aren't you. You can usually tell it's sarcastic from the deadpan way it's said.

It means to have fun. To spend time in a very enjoyable way. In context to the show it means that its fun to spend time with you and the person doesn't gets bored.

I think it means that person is full of fun or just brings fun for everyone around