To a certain degree, yes. I mean, I like it when he's not necessarily a major part of the plot, because in that context, he's a genuine show-stealer, which makes him all the more endearing. But at the same time, yes, it's almost as if he's just become an outright lamebrain.
Unfortunately, this is something that's all too common with the dimwitted characters of kids shows: they just get progressively stupider the longer a show goes on. Ed? Kel? Two very good examples.