Dean liked to play down his intelligence. It wasn't really him. You know, the whole smart thing. That was more Sam's area.
But just because he acted dumb, it didn't mean he was actually dumb. Dean Winchester was not only tall and shorter than Sam, but he was smart but dumber than Sam. Dean could've gone to college. Dean could've passed High School. Dean could've got fucking awards for his intelligence. But he didn't want to.
He wanted to be the tough kid who no one picked on. He wanted to be the kid that everyone hated because he was just that awesome. Then when he left school (Early and then got his GED) he didn't want to be seen as intimidating because he was smart.
"Sammy, Vamps nest up in Ohio," Dean called to his brother. "Cow slaughterings, I think we've got some Veggi Vamps again."
"Veggi Vamps?" Sam said. "Never thought we'd come across them again."
"Yeah well," Dean said. "They're making it kind of hard to not notice them with all the cows drained of blood and shit."
"We gonna check it out?"
"Might as well." Dean may not want people to know he's intelligent, but that doesn't mean he isn't. He could recognise hundreds of monsters almost on sight. If he's given a name he'd probably be able to tell you 90% of the gods out there. But no, Dean isn't smart. Not to strangers any way. He's dumb. Dimwitted. The exact opposite of Sam.
And that's the way he likes it.