I know of highly intelligent people with both schizophrenia and bipolar disorder, and others who are less so. I think fern makes a good point about the medication and sedation. I also think aspects/symptoms of each disorder can make a person with them possibly seem less intelligent than they are. For example, in schizophrenia, "negative" symptoms. And in bipolar disorder, depression can bring on slowing of thinking that is normally fast during stability.

One of the most famous people I know who had schizophrenia was John Nash, who was a major genius and Nobel prize winner. Elyn Saks, a university dean and professor with several degrees, also has an inspiring story (see Brilliant, successful and living with schizophrenia | Al Jazeera America)