Who is called the Queen of Courtesy in the poem "Pearl" and why?

In the Middle English in which the poem was written, cortayse has much broader connotations than the modern-day "courtesy." The word is related to "court," as in a royal court, and in the context of the poem represents the very highest nobility, dignity, graciousness, and benevolence. All of these qualities and more are exemplified by the Queen of Courtesy, the Virgin Mary herself.
But courtesy is much more than good manners; it is the grace of God Himself. According to Medieval Christian theology, the Virgin Mary, in her capacity as the Queen of Courtesy, is instrumental in the distribution of God's grace. Through her, this most wondrous of heavenly gifts is bestowed. The grieving father's deceased daughter is the recipient of such gifts; she has become part of the community of grace. The child's gift of grace was entirely undeserved, as was her father's tragic loss.