To be fair, Rarity realized her behavior in Sonic Rainboom was wrong and asked Rainbow for forgiveness which shows that she grew a bit as a character. Also when you consider that she made all those dresses for her friends for free it shows how generous she can be. She's a custom dress designer and her dresses are requested by high class ponies. Dresses like those aren't cheap and considering all that time she spent designing, creating and redesigning/creating those dresses for her friends is money lost on other projects. She probably took a huge loss during that time, but for her, it was worth it to please her friends.