I don't think it's something I would opt to do, but I wouldn't think anything of someone else doing it. Like PP's have mentioned, I'm sure it makes for some pretty adorable pictures.
For what it may be worth, the reason I wouldn't do it is because of all the PJ pants I've ever had, they've pretty much all been too long and drag the ground at least a bit. Around the house, not so big of a deal. Walking around MK all night, however, would destroy them. So I don't think it's tacky or low class or inappropriate or anything...I'm just too short. lol