The first time we flew in first class, I actually tried to look up online if there was a dress code for flying there. I was quite willing to follow any required dress code. Once I learned that there was not one, I went for comfort. That doesn't mean I was flying in pajamas, but I went with the comfortable pair of jeans and a loose fitting top that doesn't show anything when I have to bend over and grab my luggage to put it into the overhead rack.
I dress in neat, clean, but comfortable (for me) clothing. The longer the flight, the more I care about my comfort level.