Where is that in the Bible?
Romans 3:23: "For all have sinned and fall short of the glory of God..."
Mary said, "my spirit rejoices in God, my savior" (Luke 1:47).
The drawback of that from Romans 3 is that, of course, Jesus was 100% human (and 100% God) and He never sinned.
Mary’s “sinlessness” - just to be precise, is considered God protecting her from sin - not her own “greatness” in any way.
The reasoning behind this was to emphasise to the Arians that Jesus was born God — that He was God in the womb, God from the point of conception.
The various speculations that Jesus was “adopted” or conferred Godhood (different heresies choose different times when this adoption happened - some at birth, others in the temple, others at the baptism) - but the Church said “No, He was God in the womb, God from the point of conception” - and correspondingly “God’s presence in the ark signifies that God kept His entrance “ark” sinless and pure”
All of it is to point to her son - Jesus, God and Man — her role was accepting God’s plan for her