I think we discussed something similar regarding Naruto.
which is true, though Luffy doesn't seem to be so effected of it.

in anime, charecters tend to fall into the one dimensional perssona the moment we get past the first few seasons.
dumb people get dumber, smart people get smarter, eno bitches go hardcore, and crybabies can't stop whinning.

examples: as said, Homer (and the rest of the simpsons), Ranma 1/2(Ranm & Akane's realtionship), Prince of tennis (just about every single charecter, espically the shorter kids) even Berserk (Puff was semi intelgent when the series started, now he's as dumb as a rock).