Since mini-muffins are small and usually easier to get out of the pans to start with, nonstick isn't absolutely necessary, although it is nice.
I make little Yorkshire puddings in my mini muffin tin, and I think the oil heats up better with a regular finish, rather than nonstick. And you have to heat the oil in the muffin cups to get them sizzling hot before you put the batter in. That's the best way to get a good Yorkshire pudding that doesn't collapse on you. I think the heat distributes more evenly in the mini tins, too. The batter seems to cook through in mini tins, probably because the mini muffins are not nearly as dense as they are in regular muffin pans.