Catholics believe that Jesus has two natures, human and divine, but is one person, The Son, the second person of the Holy Trinity, of God.
The Son is God. God knew every human being before time began, as he planned us in his mind. Mary was always Jesus' mother in God's plans, as Jesus is God, she is the mother of God and that is their relationship in God's mind from the moment he conceived of Mary before time began.
If Mary were God's creator she and not he would be god, but we call her Mother of God, which means we admit God not Mary is God.
The title admits information primarily about God, that Jesus is God and that he is fully human, fully God, but inseparably one person.
Either Jesus is God or not. If he is, Mary is the mother of God. But note, we do not say Mary is God, because she is not the origin of God, only the origin of everything can be God, the mere mention of God in that title means Mary is separate from God and is a created being, one who gave birth to God as a human being when he came down to Earth to save us.