Is there a term in literary criticism for a character who espouses the points of view of the author? Especially, but not necessarily, if that character has no other role in the work. (This is different from a "Mary Sue".)
A mouthpiece is a person who speaks on behalf of another person, often used in a derogatory fashion. John Galt, for instance, who monologues on the subject at extreme length could be considered a mouthpiece for Ayn Rand's philosophy of Objectivism.
There's also the possibly less apt raisonneur, which Lewis Turco defines as
a "chorus character," one who performs the function of a Greek chorus in that he or she comments on the action of play, sometimes speaking as a surrogate (stand-in, mouthpiece) for the author and acting like the omniscient or ironicalnarrator of a Victorian novel
which also gives surrogate and stand-in as alternatives.