It is humor's place to mock the king, whatever throne that king may sit on. It is the way we keep things in check. Not only should it be allowed, but I feel it is necessary.
Furthermore, we are talking about freedom of expression. You have to take the good with the bad or it does't work. As a gay man, I have had to listen to plenty of jokes about homosexuals. People love to tell gay jokes and it is non stop. I do understand that these joke can be harmful. But I also understand that there is a bigger issue here and I would never try to restrict freedom of expression. That is not the healthy way to deal with it. We have to be patient and try our best to educate people using our own freedom of expression.
I'd like to highlight this, and also expand a bit from my own POV.
For me, and for others who think mockery is fine, I think it stems from the powerlessness a majority or state-sanctioned (or state-tolerated) discrimination, absurdity, and/or abuse when peaceful protest is either slow in resulting in change or if those channels make no headway at all.
These are situations in reality that - it can go without mentioning, though - are incredibly stressful for the out-groups of the religions that are able to restrict freedoms and liberties of others simply as a matter of course of their doctrine. When agreeing to disagree, taking part in voting, peaceful assembly, letter-writing, media campaigns, documentary-making, and even when taking a megaphone and blaring anger toward the mob doesn't create cultural change....the out-groups cope in various ways with it.
"Gallows humor", which will seem inappropriate by most people's standards, is a method of coping with incredibly grim situations. Those who have been silenced in the legal sphere of having the same rights as others, often utilize humor to cope.
It's better than suicide. Ya think?
Back to my bumper sticker example...if a religion is telling me that I'm weaker than a man and must hand over my autonomy to him because he has a penis, that I'm not worthy to enter paradise or heaven simply because of my worldview, that my orientation is sick and an abomination, and on and on and on....and all with a straight face and in all seriousness....then it can handle some mockery back from me as far as I'm concerned.
Call me a whore and an abomination. Act on it. Vote on it. Fine. But if you can't handle the heat, get out of the kitchen.