People dress according to the dress code advised by the ship I think that should be respected, just good manners really. If the code says suit & tie & people turn up in a shirt & slacks I consider that to be bad manners.
As far as judging people by their appearance, I think we all do that, whether we realise it or not. The old chestnut of ''putting someone in a suit doesn't make them a gentleman''...no, but at least they tried!
It's all so easy really, people know what dress codes are on the ship when they book so why not just comply with them or choose another ship? Dress is not an issue with us when we book, wouldn't matter to us if it was formal every night or casual every night, we either accept it or not.