Is Socrates a mouthpiece for Plato? I don't think so at all and I don't know of any scholars who would call Socrates Plato's "sockpuppet"--he's no such thing at all in the Platonic dialogues. Plato does not use Socrates to express his own philosophical views in most instances at all, in fact Socrates is quite frequently clearly not saying what Plato thinks. Besides the problem of trying to separate the historical Socrates from Plato's (or Xenophon's) character there's an additional, related problem of sussing out how much of what the character Socrates says is Plato, how much is Socrates, and how much is indicative of the actual philosophical positions of neither thinkers but is only being used to present philosophical problems. Quite frequently, for example, Socrates as a character raises issues or participates in debates to which there is no answer--at other times Plato clearly thinks that Socrates is wrong. In some dialogues Socrates gives an argument that seems decidedly Platonic but is phrased unusually poorly or is simply ridiculous, generally clearly intentionally so. And then we have dialogues like the Protagoras, among Plato's most sophisticated--there Plato's Socrates contradicts himself in a series of famous arguments, and launches into an analysis of poetry with Protagoras that's well-known for being completely stupid and obviously parodic. If we can nail down Socrates as doing any one thing in Plato (a troubling proposition, given the extremely varied roles he's given in various dialogues) it's not to act as a mouthpiece for Plato's position, which he sometimes opposes and rarely clearly espouses, but as a way of forcing the reader, in the good Academic sense, to do his own thinking. You're not supposed to swallow what Socrates is saying, either because Socrates is saying it or Plato is--the difference is immaterial. You're supposed to think about what Socrates is saying and come to your own conclusion.

Perhaps more relevant to the question (or really perhaps less relevant, despite addressing the wording of it directly--I really think this question rests on a false premise), Plato was not alone in what he was doing. Several of the Socratics wrote dialogues, and Plato wasn't even the first--that honor must go to Alexamenus. Antisthenes wrote Socratic dialogues (probably not unlike Plato's, if we are to believe Athenaeus), as did Aeschines (whose Alcibiades had the same title as Plato's dialogue), Phaedo (the eponymous character of Plato's dialogue), Euclid of Megara, and--most famously besides Plato--Xenophon, whose dialogues are at least partially extant. A generation, by the time of Aristotle (who himself wrote lost Socratic dialogues, which Cicero praised) the Socratic dialogue had become a standard philosophical form, and there is no reason to suppose that it was rejected by Socratic contemporaries--the Socratic school created the literary form!

To tag onto this, if I remember correctly, Xenophon also wrote about Socrates. How does Xenophon's Socrates differ from Plato's?