Una joyita = A little Jewel *

A piece of work. People called "joyita" are being ironically named by those who know the person in question is particularly unpleasant or difficult to deal with.
We have previously presented similar expressions with this meaning: "Ser un perla" o "Ser un caso", which you can read more about by clicking HERE.
Special credit here to my beloved teacher @marisacanado, who you should definitely follow on twitter for more English idiomatic expressions.

Aún no sabe la joyita que se ha llevado.
Dicen que es una joyita...
Menuda joyita está hecho el niño.