But everything else being equal (same frequent flier status, etc.), when a flight is oversold in economy and the airline needs to upgrade someone, are they going to choose the passenger in the tank top or the one wearing the nice dress or suit? You already know the answer, (but as always, it doesn’t hurt to be extra nice to any staff you should encounter).
Pro tip: Dressing well gets you better service almost anywhere you go (restaurants, not the DMV, meetings, flying, travel). I don’t wear suits when I fly, but I also don’t wear PJ’s — I make sure to wear a nice looking outfit by today’s standards and by the standards of my age group (often slacks or nicer-than-jeans pants with a button down shirt).
You would be amazed at what better service you get when you put care into your outward appearance, even if you stop short of wearing a suit. Yes dressier clothes may not be as comfortable, but planes are uncomfortable anyways. Just remember: wearing a suit and sitting in first-class will always be more comfortable than your PJ’s in the sardine seat section.