I haven't seen the episode in a long time, but I don't recall the Pakleds actually ever doing something profoundly stupid. They were extremely inarticulate, and in humans, a deficiency in verbal/conversational skills is very often a co-symptom of, or related to, other mental impairments. To put it very bluntly, if a human is very stupid, he'll be terrible at talking, and if somebody is terrible at talking (like, to the extent that the Pakleds are), it's a good if not necessarily certain bet that he's stupid in 'other ways', like mathematically.
Because of this, I think we tend to immediately assume the Pakleds were mentally impaired, but it's technically possible the entire species simply never evolved complex conversation, but that they are as smart as other races in non-linguistic ways. They seem to understand warp technology and so forth (the fact that they never invented it on their on own was attributed to laziness, I think?)
Is there any direct evidence that, just an example, the average Pakled would be worse at understanding the engineering schematics of a ship than the average human, or something of that nature? Like I said, I haven't seen the episode in years, so if the Pakleds are indeed depicted as stupid other than in the way they talk, then of course I'm wrong and you can ignore this post.