Overall, this is one the best-written fanfics I've read. It does an excellent job of telling the story from the perspective of a child with severe mental and physical disabilities. Many readers have compared it to Flowers For Algernon
, and the comparison is apt, although there is a critical difference: Derpy is never delivered, even temporarily, from her retardation in this story, and so never comes to understand what is going on around her or what is happening to her.
And therein lies one of the problems with the story. All it's emotional power comes from its portrayal of Derpy as the perfect woobie
: she suffers so beautifully. Yet some may find this emotionally manipulative on some level, because the story also denies Derpy any agency. She is totally incapable of understanding what is happening to her or that her mother is horribly abusing her. She simply suffers.
That's a further problem because this is, after all, the story of how Derpy got her cutie mark, but a cutie mark, canonically, is supposed to signify a pony's special gift or talent, and a pony is supposed to receive her cutie mark when she discovers her special gift. But this story ends with Derpy blowing bubbles with a bowl of soapy water and a stick with a ring on the end—the traditional child's toy—but Derpy does not recognize herself has having any special gift for this, nor is there anything else in the text to indicate that Derpy has any unusual talent for bubble-blowing. Derpy is unaware that she has gained her cutie mark, and, in fact, there is no indication that she knows what a cutie mark is, or what it signifies. When she describes other ponies in the narration, she characterizes them by their color (her mother is yellow, her father is grey), or their size (adults are big, children like herself are little), but never mentions any pony's cutie mark. In short, Derpy is denied any triumph in the story; she is here only to suffer, so that we might feel sorrow and pity for her.
Whether or not that constitutes a flaw in the story depends on the reader. Some will find it only enhances the catharsis achieved, but others may find it emotionally manipulative—shades of the Littlest Cancer Patient
. Either way, definitely worth the read.