But he is his mouthpiece.
It's the Pope's job to work for unity and to pass down the deposit of faith, and work out disputes among the faithful. A Pope is not divine. Never has been. He's human.
The Pope is to faithfully pass down the doctrines which we Catholics already know and are familiar with. He doesn't have his own doctrine to which we must adhere. I hope that makes sense.
If he is not faithful in passing down the true and timeless doctrines of the Catholic Church, well then, we will know about it soon enough in these modern days when much of what the pope says is recorded, and we have access to the true teachings of the Church. That wasn't always the case. If a Pope, say, two hundred years ago were to say something against revealed doctrine, chances are we would never know about it. But nowadays, we can pick apart everything a pope says. And that's not a bad thing, since the post VII Popes have all been modernists.