Huxley imagined a Brave New World where stupid people were deliberately bred as workers: "Epsilons", people without ambition, not bored by drudgery. This would be an error of policy.
If the populations could be reproductively isolated, then the Epsilons could be treated as robots. But in real life, they could not be kept reproductively separate, and their bad genes would infect the upper classes. The only solution in reality is to breed everyone to have a high IQ.
What sort of society would this be, where even menial workers were highly intelligent? It would be a more equal one, because workers doing menial jobs would be capable of doing difficult ones. So difficult jobs would be paid less than they are now, and menial jobs more.
Would clever workers get bored doing menial jobs? Wages would take care of this to a certain extent. They would be able to demand higher wages to pay for the boredom, though not as high as the difficult jobs, assuming difficulty is more off-putting than boredom. If this assumption is false (conceivable but unlikely), the boring jobs could even be paid more than the difficult ones!
Clever people would have greater incentives and ability to devise machinery to make menial jobs easier, because they would be doing those jobs themselves. They could listen to audiobooks or lectures while working. Clever people attracted to "working with their hands" would be able to do so without taking a large paycut.