What are the possible solutions for a “stupid people in large numbers” situation in a democracy?

Context

It's said that "Democracy counts the heads but doesn't weigh the brains". I think there's a book with that title but I didn't read it.

So, what if the less intelligent people organize in large numbers in a democracy? They could force governments to make bad policies.

Questions

• Is this the biggest disadvantage of democracy? Or are there other bigger ones?
• What are the possible solutions to eliminate/prevent the situation of "stupid people in large numbers" in a democracy?

Here, by the term "stupid people", I mean the commonly understood meaning. A person who's ignorant, lacks intelligence and/or knowledge, etc.