They won't really enforce the dress code unless you show up in a swim suit top or possibly those mens' tank tops with huge arm holes. We have been there a couple of times in t-shirts and jean jackets/sweat shirts. It's in the theme park afterall. No one around us was business casual dressed, pretty average theme park wear. I usually wear a Mickey t-shirt at the parks.
If you are going to a resort restaurant, that would make more sense to enforce a dress code. Within the parks, I don't really see that happening.