In a response in another thread, one individual mentioned that the virgin Mary was "the perfect non-divine human being."
I am that individual, and by perfect, I meant sinless. Which is perfect in God's eyes.Matthew 5:48
Is this an accepted Catholic belief?
"If anyone shall say that a man once justified can sin no more, nor lose grace, and that therefore he who falls and sins was never truly justified; or, on the contrary, that throughout his whole life he can avoid all sins even venial sins, except by a special privilege of God, as the Church holds in regard to the Blessed Virgin: let him be anathema." - The Council of Trent, Canon 23.
It was she (Mary), the second Eve, who, free from all sin, original or personal, and always more intimately united with her Son. - Pope Pius XII in Mystici Corporis, 110
In consequence of a Special Privilege of Grace from God, Mary was free from every personal sin during her whole life. - Fundamentals of Catholic Dogma by Ludwig Ott
Why then didn't Mary qualify to be our savior instead of Jesus?
Because Mary was not sinless on her own. Everything that is perfect in Mary comes from God. Mary would not be sinless if Christ did not die on the cross. In other words, God acts outside time, so at the moment of her conception, God used the grace of redemption merited on the cross to protect Mary from original sin by a single act of grace.
The most Blessed Virgin Mary was, from the first moment of her conception, by a singular grace and privilege of almighty God and by virtue of the merits of Jesus Christ, Savior of the human race, preserved immune from all stain of original sin. - Pius IX, Ineffabilis Deus (1854)