She didn’t fail. She was an accidental success.
From the artbook we know that Pink’s clownish demeanor was one she adopted to remain in the good graces of her sisters, and prior to that she served as a kind of assistant to them by doing unimportant tasks. Then she started demanding to be treated like an equal.
We know Pearl and Spinel are advanced, beyond mundane Pearls and Spinels from Q&As.
The Diamonds gave Pink a pair of Gems that represented for her what she did to them.
But Spinel was an astounding success, from the Diamond perspective. What did Pink do when she had work to do? She sealed away her annoying pink cling-on. But Pearl on the other hand did nothing to help Pink, actually leading her stray from Homeworld culture.
So by that metric, Spinel was a complete success. She showed Pink what having to deal with herself felt like.