"Fredo turned out to be nothing more than an innkeeper and ladies' man, the idiom for ladies' man untranslatable but connoting a greedy infant always at its mother's nipple--in short, unmanly."

This points out how Americans and Sicilians define a real man, as Puzo sees it. In America, a real man is successful with women. In Sicily, a real man takes care of his family. (Refer to Vito's statement to Johnny Fontaine, but really directed at Sonny.)

I don't think that it was Puzo's point of view, only his writing which reflected the general opinion of the time I guess.
1) Puzo's parents came from Naples, not Sicily
2) In Sicily too a ladies'man is a real man, and he takes care of his family too. And of his mistresses as well.

I think that Puzo's writing just aimed at showing how weak and "mentally retarded" Fredo was (due to an illness he caught when he was a baby). It was Puzo's way to translate Fredo's dependence on women : he never grew up into a man, he remained "always at its mother's nipple".