Being without original sin is not sufficient to be an atonement for the sins of the whole world. Anselm in Cur Deus Homo show why the redeemer had to be God. Mary was not God; she was human. And therefore, Mary could not be the one whose death takes away the sins of the world. Morover, Mary was sanctified at her conception by the grace of Christ. But the redeemer cannot be in need of redemption.
-A8
she was human.
****
So was Jesus. He was also called the Son of Man.