The Among Us hat that is a royal headpiece is the Crown. It is a cosmetic hat that places a crown-shaped royal accessory on the player's Crewmate or Impostor.
Hats in Among Us change a character's appearance without changing movement, tasks, sabotage abilities, or victory conditions. A player can wear a hat while serving as either a Crewmate or an Impostor, so the accessory provides no information about that player's hidden role.
The Crown uses a familiar symbol of monarchy: a decorated headpiece associated with kings, queens, and other rulers. That symbolism distinguishes it from a Tiara, which is generally a lighter ornamental headpiece, and from more descriptive unofficial names such as King's Crown or Princess Crown.
Cosmetics are separate from skins, visors, and pets in Among Us. Players select them before entering a game, and other players can see the chosen hat during movement, meetings, and many animations. The official item name is Crown.