That is on public grounds and would be illegal IMO.
At the very least unethical. But yes, probably not legal. But don’t let that concern you so much, the truth is you should be more worried about what some idiot(s) may do to your or your nieghbors propety if they have a front yard posted sign supporting anything other than the “O”.
Seriously, I took off the American Flag bumper sticker I had just because I know that there are REAL HATERS out there...