Maybe that's what happens in DOAWK - incorporating several ideas from the books resulted in some 'losing the context', so to speak.
I might compare it to describing a Seinfeld plot to a bewildered foreigner: 'you see, George takes a disabled parking spot at the mall, causing a girl in a wheelchair to have an accident, so George and Kramer buy her a replacement wheelchair that has defective brakes, and she goes screaming down a hill.' sounds mean, huh? it's not - it's hilarious WHEN you see it in context. When you transfer to a screenplay, you have to get the 'voice' right, or else the gags don't work. I gather that the books contain a certain cynical tone for the main character, and that the humour plays off that. The main character becomes a little mean-spirited, and the fun begins to fade somewhat. I concur with others that the film starts out promising, but tends to bog down in the middle. I saw Diary of a Wimpy Kid (DOAWK) at a Cineplex large screen (full-price), with 2 grand-daughters, one of whom has read the books (I haven't).