The storybook in the source says Sunny, Hitch, and Sprout were playing with her dolls. Sunny was making up a story about friendship but Sprout tried to turn it into a story of Rarity trying to fry the other ponies with magic. He also calls Sunny’s version stupid an boring.
Given the xenophobia that’s apparently the background of the story, I have to think little Sunny got a lot of flack from the other kids for dressing like some weirdo pegasus.