No, you don't have to be good at art to be an art therapist but it is a big plus if you are. Art therapists need to have a good understanding of art and creative media. They must be active listeners, good speakers, and critical thinkers. They should also have impeccable social perceptive and stellar judgment and decision making skills.
Art therapists, like other professionals, must have a master's degree to practice. Some states may need licensing and other credentials. The American Art Therapy Association has additional information on how to become an art therapist, as well as a list of the colleges and universities that provide accredited art therapy programs.