The appeal of Derpy - a theory.

Of course, plenty of people have their own reasons for liking Miss Walleyes, but I think I've hit on one of them. The Mane 6 (and especially Twilight) are, well, pretty gifted ponies, and privileged too - oh, not that they don't work plenty hard for their success, but they were never going to be "just another pony" from the moment they bonded to the Elements, and several - maybe all - are exceptional individuals besides. Derpy, on the other hand, is an everypony, more notable for her weak points than her gifts - later works wouldn't pile them on quite as hard as early ones, but it's clear she still struggles, even without the common appending of single motherhood. And yet despite this, Derpy is not typically shown as despondent, as the Manes often have been - rather, she is often seen meeting all of life's troubles with simple good cheer (especially when Dinky is involved). I think such a figure - nopony special and with her own struggles, but doing her best for herself and those she loves - maybe has a particular resonance with people. Well, it's just a theory...