It is not illegal to frown at a cop in NJ.
It's an urban myth that frowning at a cop is illegal in NJ, but it's not actually illegal to frown at a cop in NJ as it's protected under freedom of speech.
However if the frowning at the cop in NJ, leads to more actions that are deemed a threat or disorderly you can get arrested.
But you won't be ticketed or arrested simply for frowning at a cop in NJ or any other state.
Frowning at a cop is a form of protected free speech.
Although, while frowning at a cop is not illegal, it could get you arrested if you frown at the cop in a way that is deemed as threatening or you look like you're ready to fight them or interfere with their duties.
Simply frowning at the cop, especially from a distance is not illegal, but if you get close enough to them or in the cops face and frown at them in a threatening way or get hostile then it could get you arrested.
Making gestures of contempt, making threats or intentionally delaying a cop from performing their duties is illegal and can get you arrested.
The First Amendment protects a wide range of express, which includes verbal criticism of the police and non verbal criticism of the police.
Federal courts also have consistently held that non threatening gestures and even if offensive towards the police are protected under the First Amendment.
Freedom of expression, also includes the right to express unpopular and disrespectful views towards government officials including law enforcement officers.
Although you could also get charged with disorderly conduct, if the frowning leads to actions that fall under disorderly conduct, interfere with the officer performing their lawful duties or interferes with the public safety.
It's not generally illegal to be confrontational with a police officer, but it can result in problems or unwanted attention.
Making gestures of contempt or being overly confrontational with the police can be seen as provocative and may not be wise to do.