Before Mark got his head stuffed by Greg, we can see Mark most likely unable to move and looking at his hands as if there's something wrong with it.

In the game, we cannot see human skin textures clearly so it's unclear whether if there's something wrong with his hands, but it still implies that his hands or skin isn't supposed to be like that. Plus, they look pretty stiff.

So, did Mark's skin somehow... porcelainize/became the same material as the head?

It is pretty far fetched, and there are things contradict it, like: Mark is still able to move fluidly and he is able to move his fingers(?). But, this is a game with a walking dead guy at the end... so anything could happen.

I guess I just like the concept of Mark having his skin porcelainize, but still have his flesh and bones underneath. So that cracks can appear on his skin and revealing flesh and bones, uncanny.

Or he was just a regular zombie.