I always assumed that Mongo had 8 in both int and wisdom. He talked slowly but he did talk. His comprehension wasn't great but he did understand people. I always assumed he clothed and fed himself, and was a functioning member of the town.
Forest on the other hand had low int, but a reasonable (if not high) wisdom.
Since average intelligence is 10.5, 9 would be a little slow while an 11 would be barely above average. When you get to the 14-16 range you're starting to talk Mensa, and really, really smart. Anyone above a 16 is a certifiable genius.
The scale isn't perfect of course - while the highest IQs are around or close to 200, the lowest IQs fall below 30. But straight rolls of 3d6 do give you a decent curve that approximate the intelligence bell curve. In addition IQ tests aren't perfect because they don't measure wisdom.
Or maybe Oofta just have soft spot for Mongo (and it has been years since I've seen the movie).
Interesting interpretation.
I've also seen people argue that the 3-20 range shouldn't really be used to represent human ability but rather something like... viable human ability for an adventurer.
So Int 3 is the dumbest an adventurer could reasonably be.
This makes some sense when you do stuff like look at Strength Carry weights, which are higher for 3 strength than you'd get from someone who was literally as weak as humanly possible... Or as weak as 30 IQ is dumb, frankly.
Someone with 3 strength can carry 45 pounds in gear, or lift/push/drag 90 pounds. That's substantially above the "floor" of human strength. So, the argument goes, 3 Int would need to be substantially above the "floor" of human intellect.