Soccer moms suck, that is all.

Adding to this, Derpy doesn't explicitly have any physical or mental issues beyond the obvious: severe strabismus, or in simpler terms, lazy eye. Strabismus isn't a very serious issue (and, in modern times, can be easily corrected at an early age), and its most major detrimental effect is a lack of depth perception. Of course she'd be clumsy- she can't tell what's close to her and what's not!