True Gudfadern Vito basically left Fredo behind and almost letting him know that he is worthless...having him be some personal driver.
1. Don for Sonny
2. Senator, Governor for Michael
3. And Fredo -- well -- Fredo was -- well --

In fairness, Fredo was given an opportunity in the legitimate casino business by Sonny and later Michael but he was banging cocktail waitresses two at a time! instead of learning the business

He may have been perceived as good-hearted, weak and stupid until his kid brother surpassed him

I too reckon he was not a moron nor idiot and no doubt dangerous and deadly because of his jealousy and resentment of his kid brother, Don Michael and I reckon knew he was setting him up to be murdered for the Donship