"What is so funny?"
Translation:Vad är det som är så roligt?
Your sentence is not right but you are on the right way. A lot of people do not pronounce the last letter of a word sometimes. Of course it depends on where you come from and how formal you want to be. If I was talking to my friend I would literally say: "Va e de som e så roligt". I guess this isn't grammatically wrong when speaking and just lazy since we don't pronounce all the letters. As you see är becomes e. However you can absolutely never write it this way. As I said it is just lazy pronunciation and I wouldn't recommend learning it this way.
If you just wanted to make the expression shorter you could indeed say "Vad är så roligt?", although to be fair, the longer variant is used more often. When you say it like that you sort of put more focus on så. Try saying it in English, "What is so funny?" This is often used with 'that'. Ex:
Vad är så roligt att till och med du börjar skratta?
What is so funny that even you start to laugh?