The reason why Maira didn't like when Bruno called his father stupid is because his father was taking care of her.
Maria gets angry when Bruno calls his father stupid. She tells him that his father is a good man: her own mother worked as a dressmaker for Bruno's Grandmother. Bruno's Father paid for Maria's mother's hospital care and funeral when she became ill and eventually died, and then offered Maria and home, food, and a job in his household. She hints that she cannot understand how such a good man could be doing Father's job at Out-With, without actually saying those words.
