This is a rather odd question but the pope is the highest authority on earth according to the Roman Catholic Church, if I have that correct. But if a prophet of God showed up, would his authority be higher than the popes?
After the death of the last apostle, general public revelation was closed. That means that no new prophet of God will show up to add on to or to “complete” Christian revelation. If there were to arise a Christian prophet, he could only give private revelation, which is not intended to “improve or complete Christ’s definitive revelation, but to help live more fully by it in a certain period of history” (CCC 67). Since the veracity of such private revelation can only be rightly discerned by the magisterium of the Church (see CCC 67, linked above), obviously such private revelation would be dependent upon papal authority and could not supersede it.