I mean I wouldn't say any strain makes you dumb but I would argue that indicas have a more smack-in-the-face holy shit I'm baked (rather than high) effect.
OG Kush for sure makes me couch locked. I love it. Basically makes me 100% immobile, relaxed and euphoric.
Every strain makes you dumb. What determines it is how much you've smoked at the time and what you've smoked. If you smoke yourself into a green stupor, green out, or any of those things, then yeah...weed made you dumb, but because you went over your personal cognitive limit. Not because of weed itself. Also, if you get a hold of a real strong Indica, you won't be "dumb" per se, but you can get so tired and sleepy that the brain will simply refuse to process anything else. The flip side is, with the right Sativa and the right dosage depending on the user (very small for me as I'm a relatively light smoker), you can find yourself solving problems much easier.