I think "intelligent" is too broad of a term. They should be able to keep up intellectually with the client, and offer something the client cannot do on their own. They need to be skilled in their practice. There are many types of intelligence, and just because someone is brilliant in one regard and their therapist is not does not mean that the therapist cannot help them. If there is a big gap in intelligence in general, I think it could be difficult for the therapist to help the client, though, because the client might be steps ahead of them.