Wikia needs to get its shit straight when it comes to blog pages. Anyhow, on with the point.
I see a lot of stories now a days with what seem to be incredibly stupid characters. And I mean incredibly stupid characters. This is where you get the idea of the title: Stupid is as the Main Character does.
What's stupid about them? Let me put it this way: your character needs to be given choices. When it comes to those choices and how they are in a situation, they should the absolute stupidest decisions possible. Given that, you now have an idiot main character. And to make it particularly annoying? Tell us that the character is actually intelligent.
Now, I obviously understand that such precious stories that are near and dear to our hearts such as Sonic.exe and Jeff wouldn't have been if characters made smart decisions. But at the very least, justify the stupid. Don't leave people with a reason to ask "Why didn't he just get the hell out of there?" Your characters should think logically; remember that you want your character to relate to the audience and the audience to your character. If you have an idiot main character (who is unjustifiably stupid for the sake of stupidity) then there is an issue - you've just insulted the intelligence of your audience.
That's not to say stupidity can't make a good character flaw, but yet again, do something to justify it. Have some form of character development that tells us why a character doesn't always make the best decisions instead of having them simply make stupid decisions as the plot demands. Being a teenager who doesn't think long-term justifies them not making a decision that benefits people over the long term. It's simple, really.
That's not to say make them ultra intelligent, either. I could write down that the teen is a child genius who makes perfect grades in shcool, but I think there's a word for that. That being said, it's just not how real life works. If nothing else, take away the options. The only choice left is the choice no one would make - and now your character doesn't have to dumb down their IQ in order to progress the story.
Point in case, if you would stop and think about a situation and assess your options, your characters should as well. Stupidity is justifiable if pulled off right; but be realistic. Don't have them make a stupid decision for the sake of stupidity. Have them make a decision because it is the most logical decision they can make or because it's the only option they have.