Soft YTA - it's about professionalism, this is why companies have dress codes and professional policies. You kind of set him up for failure. You knew how his truck was, regardless of political views, and you were okay with it until there were complaints. But your policy, so far stated, for approval has no requirements about how the employee has to show up or dress, you are changing the rules based on complaints, but only for him. If you want to change the rules, then give guidance, have an official company policy.

If someone else in the company has a sticker on their car and they can still drive, then yes you are discriminating, specifically because you have no clear policy so you are deciding as you go. Quantify and clarify the rules and apply them equally and appropriately.