Why exactly does the Corleone family consider Fredo the stupid one?

The Corleone family considers Fredo the "stupid one" primarily due to his perceived lack of intelligence and ability to navigate the family's criminal enterprise effectively. Unlike his brothers Michael and Sonny, Fredo often makes poor decisions and displays a lack of strategic thinking, which leads to a sense of disappointment among family members. His failure to grasp the seriousness of family loyalty and the complexities of their operations further solidifies this perception. This characterization is compounded by his desperate need for validation and his inability to assert himself, making him an easy target for ridicule within the family.