I'd never single out some random person and mock them for their private beliefs. It would be cruel to do so, and would likely accomplish nothing. Besides, I'd have no way of knowing what their private beliefs were in the first place. However, public beliefs are a somewhat different matter. Someone who introduces his or her beliefs into the public arena is asking for attention. In these cases, a response may be appropriate.
Suppose a stranger approaches me in the parking lot of a local store and loudly informs me that he believes the Loch Ness monster is real and that scientists are concealing evidence. Whatever reluctance I might feel to mock private beliefs is not relevant here. I might opt not to mock this man at all. I might agree with him, suggest that he seek mental health treatment, attempt to steer clear of him out of fear, or any number of other responses that would not include mockery. But I may also laugh in his face, let him know that I disagree with him, or engage him in discussion with the goal of showing him the error of what he believes.