I always have a chuckle to myself when I see "Sunday best" travellers in F. I mean, really, this isn't the 1950's. Jeans/trousers and a shirt (preferably with long sleeves) are fine - as long as it will give me some protection if I have to jump down those rough canvas slides to get out in a hurry, I don't care. I do tend to wear PJ's when they are provided, because generally I do find them quite comfy, though they only go on when we are in the cruise, and they come off again at the top of descent. My shoes also come off as soon as the PJ's go on. So long as no-one has smelly feet, I don't mind them walking around the cabin barefoot. I always have my charming antiembolism socks on if the trip is over 4 hours.
I do find it slightly distasteful when passengers (in any class) flip-flop their way onto the aircraft in beaten-up trackie-dacks and a black singlet which proclaims their membership of the "Mullers and Packers Union", or that you can create an "Instant A**hole - just add alcohol". Classy. But as long as you don't get rolling drunk and become said a**hole, or raise your voice louder than required to be heard by your nearest neighbour, I really don't care how you are dressed.