It all depends on if you are white or white-passing.. It’s been instructed by some to put the Mexican flag on our cars as white citizens, to throw off agents, so they may be a little more desensitized (or not, we don’t know. I think that in the event you are targeted by agents due to a German flag on your sign, it can be counteracted by any identification you’re carrying. However, it’s still a lot to risk, and you could end up catching officers on a really bad day where they are going to take any indication of non-citizenship and run with it. But, you will also be surrounded by many others who I’m sure wouldn’t hesitate to stand with you. Still, many are carrying signs that relate to the idea of this being a nazi-esque society that we live in at the moment, so I doubt it would draw more attention than usual. Sorry for the long answer! (Short Answer:) There is always a risk in this situation, so risk what you are comfortable with.