In a mad attempt to make it not quite as painfully boring, my super rationalization sense will come up with a story that will at *least* make it all make sense.
Zecora cursed her. She's an EVIL ENCHANTRESS. Or well, possibly a good but grumpy enchantress. I mean, seriously, she wanders into town having NO IDEA what's been happening to Applebloom, and just happens to have a magic cure that can only work if Applebloom confesses her crime? That's not a cure for a disease, that's how you break a freaking curse.
That still leaves the episode having a serious overemphasis on random visual gags though.