Those kind’ve grilles look hideous. They’re usually “complimented” with a crooked license plate,which looks ghetto to the max (and is probably against DOT standards).
US DOT generally doesn't care what aftermarket cosmetic stuff you put on your vehicle after you buy it. There are some exceptions, such as lighting, as people found out the hard way some years ago when aftermarket clear taillights with colored bulbs were the rage.
But! A state might have a different take on certain aftermarket accessories.