How soon before that bumper gets rear ended? Like, really hard?
Probably have to wait a while, because most people are content to let delusional Godwin-droppers be. Now, if someone had a sticker that said, "I learned everything I needed to know about Christianity from the Crusades," that would be a whole other story.
Might as well set up an account at CaféPress.com and load that line up. Choose the all items option; coffee cups, t-shirts, bumper stickers...Because I bet that's exactly the site where the OP bumper sticker was first put up for sale. Consider it an appropriate retort. Do two opposing offensive things cancel each other? Maybe, or not, but it would be satisfying to do, nevertheless.