Most people with class don’t wear T-Shirts? I don’t know what airline you’re flying, but the ones I am on have lots of T-shirts and I wouldn’t say they were “classless” people. Out of curiousity, how old are you?
I love wearing t-shirts around, but if that's all you have on while flying you might as well just show up in your jammies and be really comfortable -
which is OK, it's a (relatively) free country, but it does look dumpy.
I'm 53, and the vast majority of my flying is for business puposes - maybe that colors my view point a bit.