Is Baba a flat or round character and why?
I think you could argue this both ways. A round character is a character who is neither purely good nor purely evil, with both flaws and virtues.
A flat character is purely good or purely evil. Generally this type of character is a stock character like a hero or a villain, or an extremely minor character in a story.
Does Baba have any clearly defined flaws? This is the central question you must answer and defend in this argument.
Baba's relationship with Amir is complex, he clearly loves his son, but at the same time occasionally fails to show his love in ways that Amir understands. Amir's jealousy of Baba's affection for Hassan makes their relationship even more complex and helps to drive the plot. The key here is deciding whether Baba's difficulty showing love to Amir is a flaw or not. Baba continually forgives Amir's missteps and never does anything to intentionally harm his son or anyone else in the story.