That God's merciful love is indeed His "greatest" attribute is implied in the Scriptures and had been taught by great saints of the Church such as St. Augustine in his commentaries on the Psalms and St. Thomas Aquinas in his Summa Theologiae.
In the mid-20th century, however, Catholic intellectuals were in the process of recovering the philosophical aspects of St. Thomas Aquinas's thought. Saint Thomas had taught that, seen from the philosophical angle, in the simplicity of God's essence, all the attributes of God are really one; they all refer to the same infinite, eternal act of God's perfect being. God does not have "parts," or attributes that are separable from each other. They are always in act, all at once, and inseparably so, bound up with each other at every moment.
Thus, God's justice is always loving, and His mercy is always just, from all eternity. Therefore, from a philosophical perspective, one cannot say that one of God's attributes is "greater" than any other; they all just refer to the same thing: His perfect being! At Vatican II, however, the Church strongly encouraged the faithful, including Catholic theologians, to recover biblical perspectives on the faith, and not just philosophical perspectives. Thus, Fr. Ignacy Rozycki, who analyzed St. Faustina's Diary for the Vatican in the 1970s, pointed out that if we consider God's attributes not just in an abstract, philosophical way, in themselves, but in their relation to us, as the Bible does, then we can indeed say that mercy is God's greatest attribute for His creatures.