Why is there a light dusting of polenta on muffins?

Answer 1

Well, since polenta is a porridge made from cornmeal, I think you are referring to cornmeal. Some bakeries use it to help prevent sticking.

Answer 2

I've never seen that unless you're referring to cornmeal muffins (instead of cornbread) and the outside might appear to look like polenta since they're both based on cornmeal.