Is YONCÉ considered a nickname? Please, really want to know
Considering she calls herself that, I would say it's fine.
The rule is more aimed towards the mangling of names. Shortforms like Yonce, Xtina, Bey, Brit, Gags are fine.
It's a popular nickname that Christina actually gave herself. She wears clothes that say "Xtina" on them and it was even her Twitter name. It's practically her name and it's universally recognized.