Part of that is because Brandon didn't have the balls to do what it's fairly obvious RJ meant to do, which was to make the readers totally appalled by Mat's decisions, and turn the characters who depended on him against him. By KOD, RJ had seriously darkened Mat. He was a more thoughtful, more sober, but at the same time his usual paranoia, his prejudices were still there. RJ had made Mat evolve from a funny trickster à la Reynard the Fox into the much darker trickters, the ones you can't be sure you should rely on or accept help from, as they might well turn around... when their tricks don't turn on you and cause the "end of the world". Mat's tricks were no longer inoffensive, the trickster was killing thousands of her own soldiers merely to open a path for his wife. Very costly. The Merrilor battle should have been more frightening, with Mat sacrificing tens of thousands for victory.

Brandon seems to have misunderstood what sort of regression RJ meant to bring Mat through. He made him return to his EF self, goofy and rather idiotic, going for the big laughs. He didn't get at all that RJ meant Mat to be suddenly less and less funny, or rather to become more and more uncomfortably funny.

Mat was another of RJ's really quite succesful characters, full of layers. Brandon ruined him, though. We got a good Perrin, a more than decent Rand, and pretty good if someone annoying version of Egwene. Nynaeve wasn't that bad, and Min. But Mat is a complete fiasco from start to finish. Elayne is close to that too, IMO, if not as bad.