I can't believe some of the answers you've gotten here OP. I personally think it's entirely appropriate for you to wear gym clothes on the plane, I have for years. I have to take 12, sometimes 14 hour flights to Asia with exhausting layovers, all the while being stuck in
overheated cabins sitting in physiologically unconscionable positions while my spinal column compresses and my muscles ache... I DO NOT have the energy to worry about my appearance to the half-drunk grandma sitting next to me or whether the stewardess will respect me to give me an extra bag of peanuts.
Moreover, airplanes are one of the most filthy and bacteria-ridden spaces in public transportation, any nice shirt or heaven-forbid sport jacket you wear in such a place would need to be washed immediately. Unless you're meeting a client on the plane or at the exit, or it's just a couple hour flight, then my hunch is the people who dress up for planes are just bored dandies in first-class who want to show off, have no sanitary sense, and have weak blood flow so they're not overheated... not to mention very judgmental about what others are wearing