I don't have a problem with jokes about stupidity (and race and religion and anything else), but to discriminate against someone unfairly for their inabilities or handicaps (or race or religion or anything else) is not something I think is great behaviour.
What do you mean by "unfair discrimination"? There exist people who are not intelligent enough to work in most jobs, because they can't understand instructions, or the tasks required, etc. If I'm an employer, that lack of intelligence alone should be enough reason for me not to hire the person. That is discrimination, but I would say it is entirely fair. As an analogy, consider this: you will never be hired as a medical doctor without the appropriate credentials, not just because it's typically not legal, but also because not having those credentials is an excellent predictor of your lack of medical knowledge. Employers need to make use of predictors to decide who to hire.
Skin colour isn't a good predictor of a person's behaviours, which is the main reason that analogy doesn't work. Ignoring for a moment the definition of "intelligence", intelligence is a good predictor of aspects of a person relevant to things like employment, and as such, I would expect employers to use this information to discriminate against these people, and I would support their doing so. Especially for a minimum wage job (where the employer cannot simply pay less legally), it makes no sense to hire somebody with severe intelligence problems. (Indeed, the existence of very low-skilled workers is an argument against minimum-wage.)
Also, assuming that intelligence is actually some intrinsic quantity, and not simply a description of a person's emergent behaviour attributes, rarely is anybody discriminated against on the basis of
intelligence: they're discriminated against on the basis of their behaviour. The only "unfair" discrimination here I could imagine would be if, for example, stupid people did not have the same rights in proceedings against them. This would be a very real possibility if it weren't for things like competency hearings and court-appointed attorneys.
MrIndigo is right, sice it is wrong to discriminate against someone for something they can't help but do.
This is a very flawed philosophy. It's not hard to imagine somebody who is genetically predispositioned to murder. If that person kills somebody, I still want him locked up, even if he "couldn't help it". Similarly, if you were an employer, would you really hire somebody too stupid to do the job, and also pay that person the same wage as anybody else? Obviously, it is not "wrong" to discriminate on the basis of things that aren't choices. The real reason for not discriminating on certain grounds is that those grounds are
irrelevant to how they are being applied.
To repeat again the earlier example: skin colour is not a very good predictor of anything interesting about a person. As such, it makes no sense to use this as a factor in how you treat the person, especially for employment. Discrimination on the basis of skin colour is wrong because it is a poor predictor of behaviour, not because people can't choose their skin colour.