There are certain life skills that can be considered “smart” like revisiting your established opinions when presented with new information, or knowing that encountering a new piece of information from a single source doesn’t carry the weight of encountering it in dozens of places, or that making a mistake doesn’t guarantee that your existence is doomed but been presented with a learning experience.
The “stupid” folks refuse to take the simplest steps to better themselves. They refuse to self-evaluate honestly. That’s the root cause. This stupidness results in them being duped and misinformed.
Therefore, when one encounters an uninformed and duped person, it’s not out of line to consider that person “stupid”.
So when people like OP say that people are stupid, it’s not a dig via victim blaming. It’s a judgement that they’re clearly being stupid, that their poor life skills need work because they ended up misinformed.