There is nothing wrong with calling Mary the Mother of Christ the problem arose when Nestorious used it to promote his belief that Jesus is two distinct persons. The title Mother of God arose in answer to that heresy.
If it is not used to deny that Christ is one person fully divine and fully human the title is quite appropriate and not heterodox.
The Chaldean Catholic liturgical books give the priest the option of referring to Mary as either “Mother of God” or “Mother of Christ”.
[Glory be to the Father, and to the Son, and to the Holy Spirit. Let there be a remembrance of the Virgin Mary, Mother of God (or: Christ), upon the holy altar.]
http://kaldu.org/14_Reformed_ChaldeanMass/PeopleBook_Eng.html