Ryngard has a nice system there, and I believe there is a table in the PHB or DMG that shows which monsters have what Int score, which is somewhat useful.
I look at it like this...He has a nice penalty on all int based checks and like no skill points, so he's basically going to make a bad decision, if making a decision, and he's basically untrained in all skills. That being said, like Forrest Gump, Int isn't everything, and with other decent stats, the dummy can still be useful/contributing in many ways.
The problem with drawing an analysis is that you have to take out all the other stats that help to make up for the low Int. You could have a low Int and average on all other stats and be completely worthless, or have low Int and high on all other stats and be fine. Hard to draw real world comparisons to game mechanics.