I never got this. If anything, DP improved on Ash by finally striking a balance between the more comedic and flawed aspects of his personality and the more serious, competent side of him. OS, and AG to a slightly lesser extent, often overplayed the former two to a point where it sometimes became very difficult to believe he could really be headed towards great things in the future. And frankly, it hurts one's ability to sympathize with him when episodes like the first Brandon battle come around and he spends it acting like an annoying, overconfident brat who gets himself and everyone around him into serious trouble because of his own stupidity (it doesn't help that Ash supposedly learned a lesson similar to this at least three times beforehand). DP showcases a more well-rounded character who still had a ways to go, but finally demonstrates consistently that he had the tools and the drive to get there.
Ash was taken more seriously in DP than prior series, yes, but episodes like DP034 and DP088 (and plenty others) show that he could still be dumb, impulsive, tempermental, and reckless. That this was balanced by an increasingly more serious, driven, and competent side was a good thing, as it helped give off the impression that Ash was growing and improving. He doesn't need to, nor should he, be a joke all the time.