Mary is the mother of Christ’s humanity not His divinity.
Saying that Mary is the mother of God is saying that she is the mother of His as God, the mother of His divinity.
That puts her above God and makes God a created being, subservient to her, which fits in very well with the blasphemy that God cannot refuse any requests of Mary, which is why Catholics pray to her. They try to circumvent God by appealing to her figuring that with motherly compassion, she’s won’t deny a request made to her by us lowly humans, and then God can’t/won’t deny her her petition.
It’s just an attempt at blackmailing God into doing what Catholics want Him to instead of saying *Thy will be done*.