No person was sinless but Jesus.
Mary didn’t need to be sinless to bear Christ.
Indeed, she had the taint of original sin by virtue of the fact that she was born of the line of Adam and Eve. Otherwise, she would have been suitable as a perfect, sinless sacrifice on the cross.
The Ark of the Covenant was covered in gold and elaborately sanctified in order to carry the presence of God.
How much more would She who literally carried the Incarnate Christ in her womb need to be sanctified, set apart, sacred and holy?