I'm going to throw in a wild guess here, but perhaps the wedding dresses weren't made by Rarity, but by Coco Pommel?



How? Rarity's perfectionism got in the way again. Two weeks from the big day and she still could not made up her mind on the design, let alone fabric. Her friends kind of saw this coming so the call in the next best dressmaker for help, all the way from Manehatton.



By the time Fluttershy, Applejack and Coco buck the door down, a frizzled and slightly mad Rarity was curled up in a ball, paralyzed by indecision.



Coco volunteered to get the dresses done in place of Rarity after she found out. The Mane 5 were a little worried whether Coco can get the two dresses done in two weeks. Coco said knowing what Rarity and AJ did for her, never mind that she owed her CAREER to Rarity, she will do it.



So they sent Applejack and Rarity on a long holiday for 2 weeks, while Coco and their friends stay behind to finish the dresses and some touch ups of the wedding, and just come back for the Big Day, now with Coco Pommel wedding dresses waiting for them.



Just like her human counterpart, Coco Chanel, Pommel is a matron saint of minimalist design. Pommel might be introducing ponified little black dress in the few years time. That helps explain the lack of frills and jewelries.