I see Christian bumper stickers all the time. And these “Christians” drive like jerks. They speed, cut you off, and even give you the finger.
So I ask you, what kind of a witness are they giving to others. “Be a Christian and you can be a forgiven jerk like me.”
Are Buddhists better? I don’t know, but Buddhists don’t need to convert or convince others. We don’t need to preach or push our religion on others. So there is no need for us to advertise that we are Buddhists.
And my advice, if anybody cares, is don’t get a Buddhist bumper sticker. You don’t need to advertise your Buddhism and it would be best if your car is offending the law or others that it not be associated with Buddhism.
This is the same reason I don’t buy Buddhist teeshirts or necklaces. I have my wedding ring and a watch, that is it. I think about what the Buddha said, “One should not make a business of Dhamma” (Ud. 6.2 Ireland).