The story for this picture involves Wallflower running away from being tormented by RD (who "decorated" her and made her into a laughingstock for the Memory Stone incident), and being pursued by Rarity who became into an angel who wants to make her into a fashion model (after all, a Christmas tree might be a good idea for the design of a dress).
Then Wallflower's guardian celestial (another kind of angel or ancient alien something) intervened and asked the obsessed fashionista to leave the dryad alone, causing the purple-haired angel to burst into tears to garner Wallflower's sympathy. The celestial Juniper later told Wallflower, to her chagrin, that she should help Rarity design her dress, due to the previous model Sugar Leaf on leave to go on a family holiday in the Bahamas.
Wallflower reluctantly accepts Rarity's choice, and the resulting dress causes her to a win a fashion gala and a lot of friends (except RD, who still hates her).