I've got a 5 intelligence Demon-Spawn Tiefling I'm going to be using in PFS. Now, since it's PFS, I could in most cases simply ignore my intelligence, as it has no effect in PFS outside of skill points and other direct numerical things.
However, I don't want to do this, and would actually like to make something fun out of the fact that my character is mentally challenged.
Now, if we assume that the majority of the common folk are generated via 3d6 methods, that means that he's dumber than 96.7% of the populace.
For Golarion, this is incorrect. About one NPC in six has a Int of 8, other than a few set-pieces, that's the lowest. In other words, folks in Golarion are NOT rolled 3d6 but use the standard array (13, 12, 11, 10, 9, 8) or Elite array.
Now, as far as acting stupid, this gets very old, very quick. Suitable for a Toon! game, but not D&D.
Also, they may be folks at your table that will be offended by this.
If they are going to BE stupid they should be played as stupid. that doesn't mean you have to be cartoonish about it. A lack of reasoning and problem solving skills, small vocabulary. not that hard and not insulting to anyone. Also reality is offensive.