The conventional wisdom is that dressing smartly conveys a certain respect for your fellow passengers. It says that you care what they think of you, whereas someone who shows up looking like a homeless bum and perhaps with similar hygiene habits clearly does not. And that tends to be the kind of person whose lack of social graces causes trouble.
I’m not suggesting that men must dress formally in a suit and tie, or women in equivalent business attire, but for heaven’s sake people, you’re out in very public places – show some small modicum of respect for your fellow humans. The lack of civility and downright fisticuffs that tend to happen on flights these days are part and parcel of the same thing.
Pure bigotry.
No, it’s the person expecting others to conform to that person’s prejudices that tends to cause trouble, by looking for anything to justify categorizing someone else as less deserving of respect.
Decent people respect others’ appearances.