>>42681543 (OP)
>>42682238
One of her main personality facets is that she's secretly very sensitive, and compensates for her sensitivity by maintaining or exaggerating that untouchable rockstar persona. Anything that betrays any of her genuine fragility/sensitivity, like going to the spa, being stereotypically girly, public failures, crying in front of other ponies, etc. are extremely threatening to her. She has no idea how to be vulnerable around her friends and show any less-than-perfect part of herself, hence her habit of independence in general, and in particular her habit of isolating herself from her friends before she lets herself be sad.
Rarity knows what she's doing with the gala dresses - her designs for Rainbow accentuate all the qualities she wants to express, without betraying any feminine fragility.