We just got off the Wonder for two cruises. They bent over backwards to make sure we were happy. On several nights, my husband ordered the ribeye with no sauce (It wasn't even on our menu, but they had it in other restaurants. And on pirate's night, they still brought it out even though it wasn't being served anywhere that night). My mom and I also ordered the baked potato that night. It was plain. I never asked for mashed potatoes, so can't say if they were plain or not.
If you want it, just ask. Several nights we weren't impressed with the desserts and just asked for some ice cream with chocolate sauce - and they gave it to us. In January, before we realized we could customize things so much, we weren't impressed with the food. This time we customized everything and we liked the food more.