Stupidity is the lack of mental horsepower. A stupid person has a weak or inefficient "cognitive CPU".

Craziness is when the output of the "program" doesn't correlate reliably with reality due to bugs in the "source code". A crazy person has a flawed "cognitive algorithm".

It seems that in humans, source code can be revised to a certain degree, but processing power is difficult (though not impossible) to upgrade.

So calling someone crazy (for the time being) is certainly different from calling someone stupid.