My thing with Rarity is I feel like you really have to go out of your way to gloss over her flaws for her to not kinda be a shitty person. I know her episodes are considered "the best" of the mane 6, but I really think the only reason people say that is because her episodes are an almost constant rise for her. She never really loses or suffers any hard setbacks in achieving her goals. Pretty much every Rarity episode has her getting the thing she set out to get regardless of what happens between the start and ending. Frankly, I find her one of the more boring characters in the show specifically because there isn't really any conflict with her. When you realize she's never going to lose, it takes the fun out of it.
But back to her personality, the issue I have with Rarity is that she was very clearly favored by the writers, and as a result the narrative allows her to get away with things other characters don't. If you actually cross reference the way she usually is compared to the worst behavior of other characters, she is kinda a bitch. She always just gets a free pass for behaviors that other characters get raked over the coals for. Even once in a blue moon when she does have to admit fault for something, she still gets far gentler treatment than other characters do. I just don't find the highlights of her personality all the redeeming compared to the low points that almost no one brings up because most of the series itself glazed over those parts.
Basically, unless the biggest thing you care about is receiving expensive gifts, I don't really find her to be all that great of a friend.