I don't like her bumper sticker either, and I find political bumper stickers to be annoying.
However, if I worked at an oil company, and had tons of very conservative co-workers, would you think it was ok for them to do the same thing to me if my wife pulled up and had rainbow/equality stickers on her car, or pro-choice stickers? I'm curious if you have a double standard on this topic due to prejudice against my wife's viewpoint (which my coworkers clearly have).
To me it's not about the political point. But about the hypocrisy of one person pushing their political views and then getting annoyed when others respond.
That's his personal property he can do whatever he wants whether you agree to it or not. It's different from literally speaking to your coworker about it.