I've never seen Derpy as mentally handicapped. Clumsy? Of course. Cross-eyed? Definitely. But never a mentally disabled pony. If that were really so, then I suppose the same argument can be made for Pinkie Pie, Clumsy Smurf, Cookie Monster and just about every other children's TV character created for comedy relief in the traditional role of the fool.
Derpy is far and away among the most beloved ponies on the show (if not the most beloved) because of her wall eyes, cheerfulness and unbridled spirit. She's different, she knows it, and loves it anyway. Dinky probably knows she has a silly mommy, but one who is protective, loving, wiser than her appearance suggests, and clearly a fit mother.
In my developing fan art, Dinky is Derpy's adopted daughter, left on her doorstep as an orphaned baby filly. Everypony is pleasantly surprised to learn what a wonderful mom Derpy really is, and her trademark ditziness only adds to her parental charm.