Changed a PS earlier this week and now was told that Narcoossee's was "casual." As Mitch said that means no swimwear or tank tops. But that's a resort restaurant of course.
No park restaurants have a dress code of any sort in our experience. Wear whatever fits the weather, your plans and your mood. Even on Christmas Day I think most folks in the parks will be rather casually dressed. If you choose to dine in a resort restaurant I think you'll find everyone dressed up a bit more on that day. At least that has been our experience.