it's a mixture of not being particularly intelligent to start with and an education basically conditioned by the wacky religious cult that took control of Ganymede (it's pretty clear that, at the beginning of the game, Jimmi is totally indoctrinated). The fact that he receives several concussions during the first part of the game doesn't help either...I think the idea is that he is extremely slow but maybe not an out and out retard. Seems borderline at times.