I teach weekend warriors and a couple guys who want to slowly work their way to a professional pilot job. I'm independent, but I actually dress slightly better when instructing in the Cessna than I do sitting at my corporate desk job. I try to make sure I have a clean collared shirt and am always clean shaven. At my desk job I often wear a tshirt and may or may not shave, as that's acceptable/expected dress in that culture. There have been many times I've come home from "work" and had to clean up before heading to the airport. Just my personal view of respecting my flight student customers.
Then again I've also flown with instructors in the past who were slobs and left me wondering if they even wanted to be flying and if they really cared about the $150-200/hour I was paying to their employer. That left me with a really bad impression, so I dont want to be "that guy".
I cannot imagine wearing an "airline" style uniform as a CFI, but I know a lot of pilot mills do it to sell the dream. The photos I see of pimple faced kids in ill-fitting shirts with epaulets loading into a Skyhawk look ridiculous to me, but whatever.
I dont care about flight suits, if someone wants to wear one for added safety, whatever floats your boat. But if they are wearing it to obtain some kind of official "look" it usually comes across as being a real tool.