It's usually used by people of low IQ. But when you think about it, "I am not" doesn't have an alternative shortened phrase, except for "I'm not" which can sound dumb in a few situations.
"I will not" has "I won't"; "I can not" has "I can't"; "I could not" has "I couldn't" and so on.
Now, "She ain't too smart" is trailer trash talk, since there is an alternative. And so is "They ain't goin' to my wedding".
Think about it...It may not be as low life as it seems.