The rise of fascism globally isn't an intelligence test. People are trapped in information silos. No matter how smart a person is, if all they have is propaganda it's rational to assume they are going to believe the propaganda. We need to get true information into these information silos to help these people.

3. A stupid person is a person who causes losses to another person or to a group of persons while himself deriving no gain and even possibly incurring losses.

This one definition covers all humans so it's not particularly useful.

Also, this implies treating life as a zero-sum game is at least a correct strategy if not outright claiming it to be the preferred strategy. Part of trying to solve our problems involves overcoming this impulse and avoiding pointless tit-for-tat cycles which often land people in the situation your argument's definition states as problematic. Namely, in game theory terms, two people cheating in a prisoner's dilemma in order to gain and thus ensuring mutual loss.

I bring all that up because this just happened in the last election where pro-Palestinian activists and Democrat supporters of Kamala Harris could not get on the same page despite sharing the same goals.

5. A stupid person is the most dangerous type of person.

We are consciousness running on meat sacks in a physics based universe. It only takes one person acting on self-destructive ideas and false information to be a danger to themselves and others. edit: typos