Taako's disguises are 100% convincing outside of physical inspection (ie, if someone tried to grab the badge it would become obvious it isn't corporeal), but are dependent on his acting. He'll disguise as an existing cop, but his information about the person themselves and the rest of the officers would be faulty enough to be suspicious.

He'd ask about "the troublemakers" and what they're going to do with them - What the sentence and punishment is going to be, in interest of trying to figure out potential consequences for himself and others.