Yes, Karishma can't multiply or doesn't know the President's name.
However, why don't we give her credit for the situations where she has used her mind? The biggest one is the fact Karishma sees Puneet's reality and figured out his strategy before most other people!
Clearly this girl isn't as dumb as everyone makes her out to be if she was able to catch on to what this manipulative person was doing. She was able to see him when he had everyone fooled into thinking he's a wise fair person and was so dominant over the house. This shows that she is indeed a good judge of character. In addition, she has consistently stood up to him.
Why do you think Puneet hates her so much? Because she called him out for his real nature of using other people's shoulders to fire his shots. Ever since he heard her say that he has hated her.