At points, it does seem like Fredo has some power within the Family. For instance, Clemenza defers to Fredo over whether to replace Paulie as Vito's bodyguard.
He's the Don's second son, and it's likely that he had some responsibility for non-vital operations.
In any case, I don't think Fredo's outburst is meant to be a literal summation of his activities on behalf of the Family.
Could be, but I think that Clemenza was simply extending a courtesy to Vito through Fredo. Since Paulie was of Clemenza's regime, it was Clemenza's way also of illustrating his reponsibility for Paulie's lapses.
It's probably true though that Fredo was more than just window dressing in the Corelone crime family. I imagine Vito tried to incorporate Fredo into the family's activities to the extent that he thought Fredo could handle them.
