A mimic is someone who is good at imitating others. A gifted mimic might be able to imitate one president after another just by minimally changing facial expression and manner of speaking.
Mimic, related to mime ("an entertainer who performs using gestures not speech"), can be traced back to the Greek mimeisthai, "to imitate." Usually when you mimic someone, you imitate them in order to make fun of them. Performers mimic famous people to get laughs, but ordinary people mimic others when they are angry at them. Who hasn't mimicked parents or a boss who has said no one too many times?!