I don't think it's mocking Christians, just those people that take certain elements of Christianity too far.
D&D isn't bad. It might lead some nutcases to do crazy, stupid things (the nerds playing it probably have greater social problems, anyway) - but saying that it's
evil is a bit much. I find it hard not to laugh at mindsets like that.
Mocking it in this way probably isn't the best idea, I agree.