To answer that, you'd have to ask whether a joke about Islam would be taken as offensive on a forum hosted and frequented by those in a Muslim-majority country.
If the answer to that is yes it would, then there are two very different approaches to humour over religion. You could then debate whether that was right or wrong.
My own personal opinion to that would be one of criticism as it is my personal belief that humour is nothing to feel guilty about and I believe God likes people to have a good laugh. There's certainly an evolutionary advantage - it boosts your immune system for a start.
Unless someone has a joke about the Koran? Seriously, go ahead...