This fits into the origin of Derpy as when they drew and animated her they made a mistake and had one of her eyes point the wrong way. This "derp" moment in the animation led to her standing out amongst background ponies, her getting a sizable fan-following, and so they honourd her origins by calling her "Derpy".
She was never meant to represent a disabled person, just someone who was generally clumsy to reflect the clumsy animation error that brought her into existence!