Part of this may depend upon what time of year you visit, but my experiences at Tokyo Disney Resort (especially Tokyo DisneySea) couldn't be further from this.
For starters, kids and teens often wear their school uniforms to the parks as a sign of pride, and these school uniforms are usually reminiscent of New England prep school outfits. Try to find kids dressed like this in the US parks.
During the spring/summer season, I observed fewer guests dressed in school uniforms or business attire (e.g., suits and dresses), but people were generally well-dressed (excepting for those guests who dress as characters or in "interesting" character-motiffs--a small minority).
During Christmas-date season, almost every guest was sharply dressed, and I'd wager that 30%+ were wearing suits, sport coats, or dresses. Of course, all of these outfits were paired with ridiculous novelty hats, but those hats actually work as a 'whimsical touch' when the rest of your outfit looks nice.
My wife and I both felt very under-dressed during our Christmas visit, and we aren't the 'XXL and gym shorts' type of guests.
I've also never experienced anything close to rudeness by other guests, unless you want to count the morning run for FastPasses, but that's not rudeness, it's just good sport!