Is it possible? Yes
Could it at least in part explain some behaviour? Yes.
But the missing question really is how much, and the answer is probably infitessimally small even if Real.
For lead exposure there are far easier and more common ways to get exposed such as lead pipes (which the US has a lot of).
But also you'd have to establish that the underlying problem is brain damage, and that is probably not true and instead reflects cultural bias.
There are many other reasons to explain American culture and behaviour which does not default to brain damage (or at least provable brain damage).
I would look at social and cultural issues first: an extremely weak political system, a poor quality general education system, high levels of religion, poor quality general health care, high levels of inequality including shocking levels of poverty.
The problem with the US is the extremes - if you have money you have the best the world can offer; if you don't then the state provision is shockingly poor. But alot of the crazies are also rich, and that comes down to the culture and society.
Lead poisoning is the least likely explanation, and is almost wishful thinking to try and explain things as a disease rather than normal human nature.