When I flew for business in my early 20s, I learned that FAs *definitely* treated passengers differently based on dress -- and that was flying in coach.
A black t-shirt and Levis would get me the bum's rush treatment at the initial greeting and beverage service -- but a suit and sharply pressed white shirt usually got me eye contact, a smile, and far more attentive and respectful service throughout the flight.
Of course, that always trashed the suit for business ... after 4 or 5 hours shoehorned into a DC9, I'd be wilted and the suit would be headed for the dry cleaners.
And that was in the early '80s ... when the planes were kept clean, you stood a half-decent chance of an empty seat next to you, and FAs served hot meals and repeat beverage service.
Today, forget it ... in coach, there's just *nothing* remotely elegant about flying. The plane's likely to be dirty, the `meal' is a pre-fab half-sandwich with Burger King condiments, and a quarter of the pax look & act like cameo characters from COPS.
And, before even boarding I'm likely to be stuck cooling my heels for an hour or two in a sardine-can gate area, further wrecking any formal outfit I'd wear.
So I'll go jeans/polo shirt or even shorts & running shoes. If I'm carrying a suit, it goes in the garment bag and arrives in decent condition.
And if I'm upgraded to Biz or 1st, you can bet I'm not gonna pass that up just because I haven't thrown on a jacket and tie.
Note: This doesn't in any way defend wearing dirty or scrungy clothes, dressing sloppily or slobishly, or behaving anything less than courteously and politely. And at airports, I've seen PLENTY of guys in $1,000 suits act like Brad Pitt on a tantrum day.


Now if ya want a dress code, ya gotta change the conditions:
Give me a widebody with something humane like 36'' and a 2-4-2, seat covers that don't look like shammy rags from a carwash, FAs who aren't so weary that they treat everyone like processed meat ... give me fellow pax who aren't refugees from Wal Mart ... serve me a meal that's actually a cut *above* Orange Julius .... and then we can talk. Meanwhile, if ya want Sergio Armani, stick to Learjets and pray for revival of the Concorde.

If you want to complain about overly casual attire, let's stick to `formal night' on a cruise ...... *that's* an occasion when under-dressing is unforgiveable.