Ryo isn't stupid; she's an autistic savant, brilliant in one area (musical composition, arrangement, and performance and history) while just kind of floating along with ephemeral understanding in other areas. Forgetting one domain of knowledge so she can concentrate on another is common, with a consistent drift to the specialization. Another classic indicator is that she is simply unable or disinclined to learn in usual ways. I'm puzzled as to why she's described as stupid.