Why would anybody dress differently for flying? Ugh, it's so gross when you see people on planes wearing sweatpants and t-shirts. Look, I'm not the most trim guy and I might also be more physically comfortable in my pajamas, but come on, a little self-respect.
Also, I have come to realize (from DDF) that a lot of us are not so makpid to look Jewish when flying (and even aderaba, which really bothers me) but I really believe that if you are out in public then you have to make an extra effort to look dignified.