Didn't Data actually say something to that effect in the episode? Everyone was making the assumption that the Pakleds were none too bright, and Data points out that they may only have poorly developed language skills.When I wrote the Pakleds, I stressed that they weren't stupid, just possessed of limited verbal skills--and overly reliant on "borrowed" technology they didn't fully understand.
It seems to be that, if anything, the message of the episode is not to underestimate people or judge them based on outward appearances.