Why do we call her " Mother of Mercy "

In short, the first reason we can rightfully call Mary our "Mother of Mercy" is that by God's special, prevenient grace, He created her soul to be the masterpiece of His Mercy in the world, and this special gift of grace within her was the foundation of His whole work of mercy in the world through Christ. Every thing about Mary was fashioned by Divine Mercy and for the work of Divine Mercy. No other creature, therefore, so completely manifests God's mercy as does Mary Immaculate.

http://thedivinemercy.org/news/story.php?NID=3369