Having high scores in INT and WIS is already represented by passing skill checks and such. It allows you to roleplay as an insightful, well learned character who knows a lot about a lot of subjects. High WIS also makes you harder to deceive and allows you to notice when you're under an illusion or not.
I wouldn't especially like a idiot dialogue just because I don't tend to find "Haha this person is so stupid!" to be especially funny. And since 8 INT is still perfectly functional it wouldn't work as being something you get from your attributes. Failing most of your INT checks already ensures your character is ignorant of a lot of subjects and that is fine for flavor.
If there was a bunch of stupid dialogue options I'd rather it be a tag you can select at character creation to represent your character just being an absolute moron. After all, even a high INT character can be really, really bad at making sensible decisions. This way if you want to play an idiot wizard you can just like if you wanted to play an idiot barbarian.
