No, I'm not overreacting. There are some things that I just don't appreciate people playing games with, and I'm not alone in that regard.
I too, would be very angry if someone stuck a sticker on my truck window.

Its a tiny sticker - 4.5cms x 5cms, easy to remove but the idea is that the action will make the driver stop and think about what could have happened to their dog.
While your intentions are good Densie, I doubt an idiot (who left their dog in a hot car)would pay any attention to that warning.