To dream that you are a mentally challenged person, indicates that you are having feelings of self-doubt. You are afraid of being left out or left behind by others.
To see a mentally challenged person in your dream, suggests that someone around you is feeling ignored or overlooked. Perhaps you have failed to listen to what they have to say.... My Dream Interpretation
Vision: Seeing a mentally ill person in a dream: it would be wise to get your lite back to a more orderly state—it might prevent serious harm.
Depth Psychology: Mentally ill people in a dream are a sign that your thoughts and actions don’t make sense. Use your head! You have a good mind! Seeing yourself as mentally ill: stay away from people with too much imagination, people that love to build “castles in the air.” See Fool.... Dreamers Dictionary