Actually, that would go a lot of the way to explaining Irkens. They need those robot back arms to be able to do a lot of basic things. Strong impetus for tool use (and hence intelligence) throughout their entire evolution. We're seeing them in a decadent phase of their history, where having tech has greatly reduced all selection pressures (sort of like in modern America), so the dumb ones aren't being weeded out.
And that explains why the Irkens are all so dumb. When you take natural selection (short of Darwin Awards) out of the picture, it stops being about who's smartest / strongest / fastest. It's who can breed fastest, and who is willing to breed at all in an environment that's passed comfortable population density. Hundreds of generations later, they're all degenerated into comparative retards. Perhaps they've long since realized that height correlates inversely with degree of deterioration. They can still expand their empire because they still have their ancestors' tech (which was so advanced that it's still ahead), but they're relying on robots to maintain it and slaves to advance it.