This is about as big a deal as if he was using Kelly Clarkson stickers instead. If somebody put a Romney sticker on my back without me knowing it, I don't see how it would reflect poorly on Mitt Romney or the rest of his supporters that didn't put stickers on my back. Even if I accidentally wore it around all day, I would laugh, remove it, and go on with my life like all of these people probably did (with or without the laughing). If a couple of people were so offended by it that they would base any opinion on people that were not involved, they were probably never coming over to this side anyway (because they most likely already were anti-Paul). Before this I'd seen a number of pranks on this guy's channel, and while most are dumb and pointless, a few made me laugh.
What you would do in that case is irrelevant. It is not how everyone else will react. If everyone was doing what you would do in that situation we would already have Paul in the white house.