If Mary had been perfect and sinless, she would have not needed a savior. Her very own recognition of that in her song proves her sinful humanity, just like ours.
As all failed Catholics well know, God preserved Mary free from all stain of sin, but as a human she needed a Savior as well, Who was her Son, Jesus Christ.
And I am certain that anyone who has been a Catholic for very long knows about Immaculate Conception.