If she really was stuck-up, she wouldn't be friends with the ponies she has. She also wouldn't be the element of generosity.
She would be cold, distant, unfriendly and not forthcoming because she would believe she was more superior than other ponies.
This is not Rarity. Rarity has a lot of self-confidence and isn't afraid to be vocal about how much she loves herself or anything/pony else. Just because she might dislike more down to Earth things, it doesn't make her a bad person.
When she says, "darling", she isn't even saying it in a condescending manner like an actual stuck-up person might say.
Basically, if you judge someone on their speech pattern, fashion sense, and preferences? You're the stuck-up one.