I wouldn't eat at an upscale restuarant at home in theme park attire, and feel that just because I'm "on vacation" doesn't mean that I'm going to "slum it".
I'm going to spend a day at Epcot with a friend in October, and we're planning on eating at Citrico's for dinner. We'll bring clothes to change into before we go. It only takes a minute, and certainly will be more appropriate for finer dining.
Keep in mind that a lot of the people eating there aren't vacationers, but locals who might be celebrating something special...
Anne