Is he mentally retarded?

He is in character.

www.kontentport.com/why-you-should-be-excited-for-guardians-of-the-gal axy-1019.html

He seems to like to stay in a character in the tv interviews I have seen with him. A lot of actors like him seem to be rather introverted and feel more at ease in front of people if they are playing a part.

lol