He got his human nature from Mary and His divine nature was always there. If He had not gotten His human nature from Mary He would not have been of the lineage of David and would not have been the fulfillment of prophesy. Of course it is beyond our capacity as humans to totally understand.
Therefore because the divine nature and human nature were inseparable she became the Mother of God.
By saying that Mary was not the Mother of God you are denying that Jesus’s human nature and divine nature were inseparable after the incarnation.
Doing so would be shear Nestorianism.
The lineage was from Joseph, not Mary.
Luke 1:27 27To a virgin espoused to a man whose name was Joseph, of the house of David; and the virgin’s name was Mary.
Jesus could not have gotten His divine nature from Mary else she would be God.
A human being cannot produce or generate a divine nature.