Yes, if there is a gym on the plane.
Good answer!
But seriously folks - everything in moderation. My flight attire can vary widely, depending on when I'm traveling, who I'm with, where I'm going...
My normal standard for flying would be something like a polo shirt and nice jeans. However, I often wear my nicest shoes on the flight rather than risk them getting crushed/scuffed/whatever in my baggage, so if I'm going somewhere I'll need nice shoes I am forced to up my game and wear nicer clothes on the plane.
If I'm traveling with a client (often the case for any business travel), I'm going to increase my formality level by +1 (+2 for a C-level or equivalent). The jeans will be khakis or even gabs, polo switched for a button-down, and quite possibly a tie+blazer as well.
Conversely, if I'm taking a red eye, that is usually a -1 to formality. Most likely I'm going to be tired and a little grumpy. Clothing will be optimized for comfort and getting through security with a minimum of hassle. T-shirt or polo, jeans, shoes that can slipped on and off quickly.