If Mary’s his mother, is she not the mother of God? I don’t think they’d have to call her anything but his mother, that should be obvious.
One must twist meanings for words to make the false conclusion you raise. In human terms a mother is a progenitor for the gestating being. Jesus existed before Mary, so she cannot be His progenitor, but she can be the Mother to His human body for His Embryo gestated in her body. Thus she is The Blessed Mother of Jesus..
The lines begin to blur when you call Mary the mother of God.
Is she the mother of the Father? The Spirit?
Scripture is clear on how Mary is referenced....His mother.