Nah, you're not.

I've never heard of cussing (or lack thereof) being related to intelligence. People who cuss choose to cuss; it's a restraint thing. So choices don't make people smart or dumb. For example, I don't think I'm dumb, yet I make stupid decisions when manic. I even make stupid decisions when depressed because I'm in a "just leave me alone" mood and will do anything to make people leave me alone. That doesn't mean I'm dumb; it means I have poor restraint.

Having restraint is good. I think it makes you smart and aware.