Did the Blessed Virgin Mary merit to be the Mother of God?
Was the Blessed Virgin Mary worthy to be the Mother of God?
First, merit by any mere human person, even the Blessed Virgin Mary, always begins with the free undeserved gift of grace. No one can merit the prevenient grace of God given at the time of sanctification (at Baptism, or for the Virgin Mary at her Immaculate Conception), nor can anyone merit the prevenient grace of God that goes before each act of cooperation with grace. However, we can all merit by continuing in a state of grace and by cooperating with subsequent grace.
So the first point is that the merit of the Virgin Mary is limited; it is dependent on the prior free unmerited gift of grace.
Second, there are three types of evil: moral evil (sin), physical evil (harm or disorder or imperfection), and metaphysical evil (finiteness). A person can be worthy or unworthy in any of these three ways. Mary is worthy to be the Mother of God because she is sinless, and because she is perfect. However, she is not worthy to be the Mother of God in the sense that she is finite, and God is infinite.
Merit occurs by avoiding sin, and also by doing what is more perfect. And in these two ways Mary merited as much as possible, since she is sinless and perfect. However, she did not merit on the third level, finiteness versus infiniteness, since she is finite, but God is infinite.
So Mary merited to be the Mother of God, but her merit of that role is limited; it is dependent on the prior free gift of grace, and it is limited by her own finiteness.