Clinical psychologists do have more supervised training in therapy (and some in research too) before they actually practice solo, so that gives them an edge over social workers or counselors. It is also much harder to get into a doctoral program than a masters program, so you will have to do less weeding out of not-so-bright people. Not that masters level therapists are not intelligent, most are. Some people just don't have the time or resources to pursue the higher degree. Still, there are some who aren't the smartest people. In doctoral programs, you have to be very intelligent to make it through. They might not all have great people skills, but none are dumb.