My two cents worth!
from what was seen onscreen, I don't recall anything that Fredo was stupid
He
knew exactly what he was doing
every time
1. Vegas outburst
2. complicit in Tahoe shooting
3. brothers having drinks in Havana
4. deliberate and deceitful concealment of the Senate hearing
5. cunningly withholding information
6. boathouse outburst
No doubt even otherwise stupidity, shortcomings is no excuse for betrayal of the family and setting his kid brother up to be murdered
If he had volunteered the information he knew about the Senate hearing even after his betrayal then "redeeming feature." perhaps could give him the benefit of the doubt but he cunningly withheld it, for Michael to commit perjury and sent to prison
Fredo's taking sides was no more a violation of family loyalty than Tom telling Sonny to negotiate with the five families rather than attack them.
apples and oranges!
Fredo taking sides against the family was always for something in it for me -- on my own.
Tom telling Sonny to negotiate was business and a Gangland war, they were unlikely to win