Ron wasn't unintelligent to begin with, he simply wasn't the best in school. He preferred socializing and sports over studying and didn't go out of his way to learn like Hermione did.
In my opinion, his low self esteem hindered his education the most. He was the second youngest of seven, he didn't see the point in trying to be the 'best' at anything because one of his brothers would have done it first. So, he just didn't apply himself more than he had to, especially to things he didn't like (like school work).
I know plenty of people, myself among them, that are smart but poor students. Ron still managed to be a strategist, a rebel, and a great friend, for all he didn't read.