"You think it is funny?"
Translation:Ти думаєш, це смішно?
why isn't there a що here? This is a subjunctive so you can use either a comma or a що to combine the parts right?
It is a subordinate clause detailing the "message" of a verb of thought/speech. Omitting "що" in such sentences is quite common, though not as common as omitting "that" in their English equivalents.
A comma is always required here by Ukranian punctuation (it is a subordinate clause either way).