What is the correlation between smartness and niceness?

There is no absolute, "one size fits all" correlation that one could ascribe to all of the characters. Lennie is fun-loving and nice, and he is the least intelligent character in the story. One can not really say that he is not nice when he smothers a mouse or Curley's wife because he has no real control over his emotions and his body in those panicky states. So, with Lennie, his mentality correlates niceness with lack of smartness. George is nice deep down but has a tendency to behave in mean ways. And he is smart. We could correlate intelligence and behaving in an acute, mean way even though this ignores the fact that George is generally nice with a mean exterior. But for the sake of simplicity, George is smart and can have mean behaviors. This suggests a correlation between intelligence and being mean. This works well with Lennie's behavior which is unintelligent and nice. Again, this has to do with behavior and it ignores George's generous spirit and occasional nice demeanor. But the simplistic correlation, using the behaviors of these two characters fit: nice/unintelligent, mean/intelligent. 

However, it doesn't take long to show that these correlations do not apply to everyone. Curley is mean and stupid. He may have some intelligence but there really is no hard evidence of it. He has a job because his father is the boss. He doesn't take other people's feelings into consideration. If he has intelligence, he doesn't use it. Slim is nice and intelligent. This defeats the idea that intelligence must correlate with meanness or that the unintelligent must correlate with being nice.