The book gives another side of Fredo. Rather than a dim-wit, I think he was made out to be more of a "not serious enough" playboy kind of guy. The book also makes him out to be tall and handsome I believe. His weakness seemed to be more a weakness of character than a mental or physical weakness.

Making him out to be a dim-wit in the movie enabled us to more quickly understand why he was passed over and how he could have been vulnerable to outside influences without going into alot of detail.