In most states it's a violation to display anything visible from outside of the vehicle that is a significant distraction to other drivers (DMV/cops have discretion in that regard). For instance profanity is technically a violation. Depiction of nudity or any sort of obscenity are also violations. 

The same goes for excessive tint on windows. They won't pull you over for that alone, but they will cite if you're stopped for something else. 

Many people do not know this - there are a lot of products and services sold to people which are illegal to actually put on a vehicle. But the people who install or sell them have no liability, only the person to whom the vehicle is registered and/or the driver.