I'm still rooting through all these interesting topics and threads.
I think Ennis and Jack had enough smarts for the time and place -- it's too bad our society is so removed from natural wisdom and physical competence. I couldn't do the stuff they did, nor could many men I know. Finishing high school would have been good for Ennis, but college wasn't so necessary then. (I didn't go to college, and I'm nearly their same age.) I see Ennis as very competent (and that is extremely attractive and sexy, too). Jack is a great people person, so his work as a farm equipment salesman was suitable. Who cares if he can't do the books -- lots of people can't. Ennis, though, was held back from better employment not because of intelligence but because he couldn't commit to a more serious job.
Anyway, I don't see anything "dumb" about them, nor was it a foregone conclusion that they were "going nowhere." Just little twists of fate could easily have brought them on whole different paths. As to their language, they probably wouldn't have talked so slangy if they'd grown up in a big city, for instance.
Bottom line is, smart enough for me!
kathy