In "Samaritan Snare," Pakleds "play dumb" in order that the Enterprise send over its Chief Engineer to help with repairs. Quite possibly the Pakleds were truly dimwitted, but it seems equally likely that it was just a ruse, in order that unwary, generous crews send over personnel to help---only to be tricked into a hostage situation.
You've hit the nail on the head. Based on Troi's perception that the Pakled are "insincere" and not really wanting technical assistance, we can reasonably assume that their malfunction was simply a ruse (a snare, if you like) to trap unwitting helpers (Samaritans) into lowering their defences.
As to whether they're actually dumb, the answer is a qualified yes. The official StarTrek.com page for the Pakled describes then as "slow-witted" but by no means as simple as they seem. The implication is that they make up for their lack of raw intelligence with ambition and deceitfulness:
A rotund, lethargic scavenger race who nonetheless are not as simple as they seem. Their "found" technology has accelerated their abilities at a far faster rate than their understanding and maturity can apparently handle
The slow-witted traders often frequent Deep Space Nine but little is known about them in the Federation. The Pakleds have met at least the Romulans, Klingons and Jarada. They are so deceptive it is not clear if Sector 006 is truly "far from home" for them
The Star Trek Encyclopedia (considered fully canonical) goes even further, describing the Pakled Captain as intelligent, devious and possessed of considerable cunning.