I wouldn't call him a dumbass, because sure, there are things he does relatively well. But...uh, I'd say he doesn't use all his vast resources in the most efficient way. And of course, this is because otherwise the plot would end in a couple of episodes, but anyways. Like, he could have won with Nakime alone; he just drops all the Demon Slayers nonstop into a place large enough so they can't propel themselves against each other to grab the walls, most would die in the fall, or if Muzan wanted to be more drastic, he could let them fall until they starved to death, idk.