Calling Mary, the Mother of God puts her over God. And God bows the knee to no one. Therefore the statement is Blasphemous.
It's a statement about that's more about the PERSONHOOD of Jesus Christ than Mary herself, that there are not two persons in Jesus, but only one, with two natures, human and divine.
Also, do you deny that Jesus was subject to Mary and Joseph?
"He went down with them, and came to Nazareth, and was subject to them." -Luke 2: 51