why it's wrong with" what is so funny"

Your question is a bit unclear so it's a little hard to answer.

As far as I know, there is nothing wrong with asking "what is so funny?".  You can also say "that is very funny", but that is not a question, but a statement, and it means something else. When you're asking "what is so funny?" you're trying to find out why something is funny. When you say "that is very funny" you're declaring something as funny.

I hope this answer helped. If I misunderstood your question, feel free to clarify. :)