That sounds plausible to me. I don't have a definition of "cleverness"/"intelligence", but I could believe that in many species most of the neurons are working

That sounds plausible to me. I don't have a definition of "cleverness"/"intelligence", but I could believe that in many species most of the neurons are working on processing sensory data or controlling muscles, which isn't the essence of (human) intelligence: you do call a juggler "clever", but a human who is blind and mostly paralysed isn't less "intelligent". I think it has been suggested that the "intelligence" of octopuses might be a side-effect of the huge amount of data processing that is required to control a body without a skeleton.