ADHD is very common, and you would not be discriminated against for this reason. However, if your “explanation of lower grades” came off sounding like you were making excuses, that would be a problem. Also, as @blevine said, the essay is your opportunity to reveal the things that make you unique that will give colleges an idea of the value that you will add to their campus. The ADHD/grades explanation does not belong there. If you feel that you would like to have this issue addressed, better to have your counselor address it independently.