Cloning is a difficult procedure to create an organism that's (mostly) genetically identical to a different organism. It will no doubt become easier in the future, but will always require more effort than the regular means of procreation.
Why do you think this lends itself to the creation of "slave classes"? There is currently no shortage of stupid people. In fact, the lower your requirements, the wider the selection.
If you're talking about "below-normal" intelligence - if we ever find ourselves in a society where it's legal to create slaves of purposefully sub-human intelligence, then we'll have much bigger problems to worry about.