I don't know how much schooling Juanqui and Ferru completed, but they are far from dumb. Juanqui has multiple business interests and Ferru reads a lot and from a wide variety of sources. Youzhny, Tipsy and various players on the women's tour show me it's possible to overthink things. I remember Youzhny being quite a headcase, smashing himself on the head with his racquet and drawing blood.
At the bottom end, as far as I know, is Fernando Verdasco, who quit school at 11 to concentrate on tennis. One source I read said he was diagnosed with ADHD. I don't know how true that is, but it's not an excuse for his crass on court behavior.