the hilarity of this stunt depends on how amused you would be to find that someone had defaced (vandalized) a bumper sticker on your car.
As far as I'm concerned, any sticker I place on the outside of my vehicle becomes part of my vehicle, and, as such, any defacement of the sticker is equal to vandalism to my vehicle.
So, yeah. Run around vandalizing other people's property if that's what floats your boat. But don't whine about it if someone does it to you, or if you get caught and the car owner decides to press charges.