I think it's important to consider why you think this. Try and explain what makes someone stupid.

I do tend to agree with the general statement that most people are pretty fucking stupid. If IQ were a meaningful number of intelligence, I'd wager that it's heavily skewed left. Meaning that the common saying of "think of how stupid the average person and realize half of all people are below that" is even worse when you use the median.

For me, what makes someone stupid is lack of curiosity, lack of drive to learn, and lack of critical thinking. I think stupidity is a learned trait, and our modern society is doing its damnedest to make sure children learn it as soon as possible. Never question authority, you only need to memorize so you can pass the test, and you will be spoon fed the information.

Then soon as you get out of school, you have to get a job and occupy most of your time with work or sleep, you'll likely get only two-three hours of time to yourself each day, meaning you'll lack the time to break out of the cycle. And the system compounds at most jobs. Your manager is likely stupid, meaning they want you to never question authority, just do what they tell you, and ask them very little questions.

I also think the trillions of dollars that are spent on advertising strongly influences this. And being constantly bombarded with psychological manipulation encourages stupidity.

I also think stupidity is compounding in and of itself. The less you know, the more you can just make hasty assumptions, then use those assumptions as fact for your next set of assumptions.

It's also contagious. Being around people who are less stupid than yourself makes you feel bad, so you aren't around them much or encourage them to join you in being stupid.

There is a massive difference between not knowing something, and choosing to not know something. Just about every person in the world has access to the greatest source of information that has ever been created. There are free courses on just about every topic you could ever desire to learn, fingertips away.

There is also a massive difference between knowing something and rote memorization. Being able to follow the logical chain of facts is very important, so is being able to critically think about a topic. I think being "bored" is great at combatting stupidity in this way. Spending time with no stimulation is great for engaging your brain in actual thoughts. Consider dedicating time to just thinking: no audiobooks, music, podcasts, video games, movies, TV shows, social media, books etc. Just sit and be bored for a while. Meditation is a great entry into this.