In some sense bans on lead are mandatory treatments for stupidity. The same goes for government-mandated addition of iodine to salt.
It makes me sad to see non-iodized sea salt become trendy in the kinds of circles where vaccines are considered "unnatural" and kids get whooping cough.
I think there is a general issue here where "libertarianism" and "paternalism" come into conflict.
My preference in nearly all such cases is to default people into the thing that seems to honestly be the best policy, and let people opt out in a way that involves some larger or smaller trivial inconviences if they want to be contrarian for some reason.
It's great to make people more aware of bad mental habits and encourage better ones, as many people have done on LessWrong. The way we deal with weak thinking is, however, like how people dealt with depression before the development of effective anti-depressants:
The only "anti-stupidity drugs" we have are nootropics. But the nootropics we have weren't developed as nootropics. Piracetam was, I think, developed to treat seizures. L-DOPA was developed to treat Parkinson's. No one knows who started using ginkgo biloba or what they used it for; it was used to treat asthma 5000 years ago. Adderall derives from drugs used to keep soldiers awake in World War 2.
And none of them are very good against stupidity. AFAIK, to date, not one drug has been developed by understanding and targeting the causes of different types of stupidity. We have the tools to do this--we could, for instance, sequence a lot of peoples' DNA, give them all IQ tests, and do a genome-wide association study, as a start.
We don't research these things because society doesn't want to research them. People don't conceive of stupidity as a disease that can be cured. We need, somehow, to promote thinking of stupidity as a mental illness. As something drug companies could make billions of dollars off of.