I'm assuming that stupid people are louder and more expressive than rational people. Because the less you know, the more certain you are of being correct, they tend to be very passionate about their own views. On the other hand, the more you know, the more you know that there is more to know, so you are less certain of being correct and less passionate about your views.
Combine this with "exponential megaphones" (e.g. internet in general, especially social media) and you have a death spiral of stupid, angry people filling the majority of online social spaces.
I Am Misperceiving an Expanding Stupidity And Here’s Why
"The Social Dilemma" documentary argues that various disinformation, polarization, and alike campaigns are potent weapons against countries (especially democracies), as well they have become dirt cheap with rise of social media. It further points out that there are traces of those already being employed in various countries in last years.
HYPOTHESIS: stupid content (or amplification of it) is mis-attributed to general stupidity that's propaganda instead. Sadly it's contagious and can lead to real stupidity, which at worst will trigger a death-spiral.