So...it was just culture shock that convinced him the guinea pig he just ATE was giving its own life to trick him?
And I know there's a culture shock issue, but after ten years (at least) on Earth, he could have learned the basics. His schleppiness and general weirdness may make him look less "on the ball" than he actually is, and in a culturally unbiased test he may score slightly higher than Fry (if there's a food prize involved), but I think we have to conclude that he still isn't the brightest spoon in the silverware drawer.