Applying a bumper sticker is absolutely vandalism. It mars the paint finish. Paint on cars is surprisingly delicate, and is easily damaged by mechanical means

Applying a bumper sticker is absolutely vandalism. It mars the paint finish. Paint on cars is surprisingly delicate, and is easily damaged by mechanical means, such as by setting objects on the roof. The bumper sticker itself won't hurt the paint, unless you leave it there for a long time, but trying to remove it is very likely to.