I never liked how in the movie they made Fredo look so weak. But I guess it was the smart way for FFC to go since it would tug at our hearts alittle more when he is "taken care of."
I woulden't say Fredo was stupid. Like you said he was smart with woman (which is a very hard thing to be smart about) and very good with people. The way he entertained everyone in Cuba was great. Micheal coulden't have done that.
Actually the only stupid thing Fredo did was betray Michael and think he could go unpunished. Oh and marry Deanna, but I'm sure he had fun with her while it lasted.