Where does the Blessed Mother acknowledge she is a sinner?
The Magnificat
My soul doth magnify the Lord:
And my spirit hath rejoiced in God my Savior.
Because He hath regarded the lowliness of His Handmaid:
for, behold, from henceforth all generations shall call me blessed.
For He that is mighty hath done great things to me: and holy is His Name.
And His mercy is from generation unto generations, to them that fear Him.
He hath showed might with His arm:
He hath scattered the proud in the conceit of their heart.
He hath put down the mighty from their seat, and hath exalted the lowly.
He hath filled the hungry with good things: and the rich He hath sent empty away.
He hath received Israel His servant, being mindful of His mercy;
As He spoke to our fathers, to Abraham and to his seed forever.
"My spirit hath rejoiced in God my Savior."
To rejoice in her Savior, means she acknowledges she is a sinner in need of salvation, i.e. needs a Savior to save her from her deserved destination -- Hell. (Where we all would be BUT for the saving Blood of Christ.)
To be saved implies there is something she needs to be saved from. If she was sinless, she would not need the Savior and therefore her spirit would have no need for rejoicing.