I generally try to dress in clothes that are comfortable but still a bit nicer looking. There is a balance. For long haul flights I occasionally change into something a bit more comfortable or just throw on a big sweatshirt. I personally wouldn't want to head onto the plane dressed looking like I'm in Pajamas. Just flew to TLV recently with a plane full of folks in sweats , T-shirts and their ugh slippers. Seriously tons of frum Jew's looking like it was their uniform.