There are two main features of Mama Day that imply it is designed as a tragedy: First, the catastrophic climax of the novel is the ultimate conclusion of George's hubris, namely the hubris that helped him to believe he knew what was true and what was false about the universe, just because he was a fair scientist. The novel suggests that actually, the universe is mostly mysterious. The effect is that the novel disagrees with George's unfounded belief in materialism. Second, the novel is a tragedy because it evokes pity and fear. The death of George's wife evokes pity, and the fact that George's arrogance and pride caused that death makes the reader feel fear for their own safety, given that everyone suffers from arrogance and pride.
Look for instance at the strange mysticism that Mama Day represents. To George, she seems crazy, no doubt. To Cocoa, the mother represents her real mother, with all the difficulties and nuances that implies. But in the context of the narrative, her role is far different than either of those roles. She represents George's opportunity to be successful through humility. If George had humbled himself enough to take Mama Day's advice, he would surely have saved his wife. Therefore, Ruby represents the opposite force, the curse of death and failure.
This balance represents the missing element in George's character. As a firm scientist, George has walked away from the unknown by pretending he understands the universe. The effect is that when he encounters the unknown, he fails tragically. A better George would have been a George who believed he understood some of the universe, not all of it. This would have readied him to encounter his limitations successfully.
