"Stupid": You say that like it is a bad thing

If the author of The Bell Curve is correct, then there are sub-populations within the United States that consistently average below the mean for the US population.

There are also some groups that average above that mean.

This causes endless anguish among the "intelligent".  To them, it is the cruelest possible fate...somebody who is irredeemably Not-Like-Them.

Let's look at the issue from the other direction

A year with a 10% lower-than-average rainfall is considered a drought an every five year event.  Two years, back-to-back of drought are an every twenty year event.  Three years in a row of drought is a once a century event.  Locally, rainfall can vary by much more than shown on this chart.

Let us speculate that under certain conditions a "blunted" intellect is favorable to survival.  One such condition is when resources are scarce.  These conditions would have to be severe and long term (many generations) or recurring (like monsoon failures) for this condition to produce a durable change in the sub-population.

Since "intelligence" is a complex phenomena with low heritability,  "severe" means multiple occurrences of +50% mortality and the loss of many family lines.

If all regions of the world, every village, every ecosystem, had the same selection pressures on "intelligence" that created college professors, we would all have (less those of us who suffered brain trauma) vocabularies of 60,000 words and I.Q.s of 140.  That is clearly not the case.

Whatever else you can say about humans, we actively change our environment to favor ourselves.  Consider Finny in John Knowles' A Separate Peace.  Finny is a "super-human" and he creates a game that is perfectly attuned to his every strength.  Gene shakes Finny off a tree branch and Finny breaks his leg.  Gene recognizes after many decades it was due to frustration and spite.  Finny could no longer play Finny's game after his leg was broken.  They both changed the environment to favor their unique profile of strengths.

How would a population reconfigure an environment such that it favored a population with an I.Q. of 85, a population that did not process most things at an analytical level?

It would be faster moving.  It would be violent.  It would be tribal and the tribal markers would be grossly large and flashy.  It would be resource starved.

"Stupid" (or rather, not "intelligent") is pro-survival in those environments.  And that is intentional.