Not a decal, but one accessory that I definitely HATE are those ball-sacks that guys hang from their diff or tow hitch. I'm not going to post a pic because I think it's that ridiculous - makes me want to carry a pair of scissors around and get rid of them whenever I see them. So redneck and vulgar IMHO, and that's coming from someone who grew up in rural NC and appreciates dirty jokes.
Carry a little can of robin's-egg blue rustoleum and give em all permanent blue balls when you see them.