He is God but Mary is not the mother of the Father or the Holy Spirit. Hence the reason she’s not accorded mother of God.
The title "Mother of God" does not mean that she is the mother of the Trinity nor or the divinity of Jesus. Actually, this title is more about the person of Jesus than it is about Mary. There was an early heresy that Jesus was two persons, one divine and one human. The title "Mother of God" or Theotokos is used to show that Jesus has two natures, one divine and one human, but is one divine person. Since Mary is the mother of the one person who is Jesus, who is God, then she is the Mother of God.