Actually I think Rarity is fairly smart since she is good at designs and using the precise measurements needed for her customers to fit in the extravagant clothes she make. She also likely have to schedule her clothe commissions for her customers as well as make a sensible amount for those who comes into her shop. She also likely have to fifth adjust any that her customer may choose off the shelves. She is also very creative, likely to also have to often check the trends to see what appealing to the wide cast of individuals who would very likely be interested in her clothes as well as adjust based on their interest but not let them completely decide how it turned out like the time her friends and up doing that. She's also can be fairly mature since she handles her relation with her sister fairly well as well as Spike who she know he have a crush on her.