Regardless of whether such a code is or isn't appropriate in a theme park, personally, I would be extremely uncomfortable if I was inappropriately dressed, even if I was welcomed in "as is", to the point that I wouldn't enjoy my meal.
I don't think that Disney Dining, the concierge or anyone else would take the time to point out a dress code if they did not intend to enforce it (and I would be rather disgruntled to take special pains to dress for a nice evening meal, only to arrive and find I'm the only one dressed appropriately). Thus, when making restaurant choices, I'd either plan to return to my hotel at some point and change, or find an alternate restaurant.
Taste in clothing is rather subjective - many clothing items simply cannot be pigeonholed as acceptable or not acceptable. While a lady wearing a silk top with spaghetti straps with a pair of coordinating culottes and high-heeled thong sandals could be considered appropriate, a woman wearing a sweaty tank top, short shorts and flip flops wouldn't be, even though both could be perceived as wearing tank tops, shorts and thongs. Therefore, it's difficult to pigeonhole one clothing item as appropriate or inappropriate. As always, good taste should prevail. Surely I'm not the only person whose mother taught me there's a time and a place for everything (and trust me, I'm a WAHM... I've been known to work in my jammies, but that doesn't mean I'm going to drive to my daughter's school to pick her up while wearing them!).
