Fr. James McTavish examines the question in detail in a recent HPR essay, "Mary, Mother of Mercy. Christ the Power of Merciful Love":
We have many popular prayers that speak of Mary as Mother of Mercy. For example, “Hail holy Queen, Mother of mercy” … and, later: “turn, then, your eyes of mercy towards us.” In another prayer, the Memorare, we hear: “To you I come, before you I stand, sinful and sorrowful, O Mother of the Word incarnate, despise not my petitions but in your mercy, hear and answer me.” Moreover, in the lives of the saints, we hear Mary referred to as “Mother of mercy.” Once, St. Maria Faustina Kowalska had a vision of the Blessed Mother. Mary said to Sr. Faustina: “I am not only the Queen of Heaven, but also the Mother of Mercy, and your Mother” (Diary of St. Faustina,330). Also, Pope John Paul II refers to Mary as Mother of Mercy in his 1980 encyclical, Dives in Misericordia:
Mary is also the one who obtained mercy in a particular and exceptional way … Mary, then, is the one who has the deepest knowledge of the mystery of God’s mercy. She knows its price, she knows how great it is. In this sense, we call her the “Mother of mercy” (§9).
Note that the Hebrew word for mercy, rahamim, comes from rehem, a mother’s womb. God’s mercy must have something of a maternal warmth about it—unconditional, intimate, and nurturing love, symbolized by a mother’s womb. God’s mercy is, therefore, tender and affectionate, life-giving and indispensable. God chose Mary to reveal this merciful love to us. Therefore, we need Mary in our lives as Christians. Many people, many sects, will tell you that you don’t need Mother Mary. Have you heard that? One elderly Jesuit I know told me once that some people were arguing about how we don’t need Mary in order to know Jesus. They told him to go directly to Jesus. He replied that God has given us Mary for a reason, suggesting finally: “Okay, you go directly to Jesus; I will go through Mary. And we will see who gets there first!”
God has given us a mother for a reason.
Read the entire essay at www.HPRweb.com.