We'd have to define "dumber" because adult literacy and numeracy rates have largely increased globally since the 1990's.
In already developed countries the Flynn effect seems to be reversing, ie IQ is leveling off or even dropping.
It's basically impossible to improve an adult's g-factor, for example. For children, things like nutrition and hygiene (e.g. no parasites) play a big role. But the kinds of things a human learns or tools they use doesn't significantly affect G-factor.