I learned a lot from the comments. Mockery of Mormonism is widely agreed to be hilarious because it is "obviously" absurd. For example, it is "obvious" that Joseph Smith is a con artist. When a similar treatment is aimed at mainstream Christianity, exposing the "obvious" lies, it seems tacky and offensive because these lies are widely believed.

That said, I can't tell if the original post was designed to troll people who would be offended by its style, or designed to amuse people who already agree with its message.