Sure, but we don’t go around calling people with ADHD mentally ill, cough. The same way we don’t go around calling guns “mass drivers”, even though that’s exactly what they are.
I'm diagnosed with ADHD, I self-describe it as a mental retardation. Mentally ill seems accurate, too. I don't have a learning disability, per se, whereas most forms of mental retardation involve a learning disability. Perhaps that's why people aren't so fast to call ADHD a mental illness or mental retardation.