1. What does it mean?
닉값 adapts the idea of 이름값 to an online nickname. Someone whose username predicts their behavior is 닉값한다; someone doing the opposite may be said to fail to live up to it.
Also written or said as nikgap
2. The nuance
It is usually a playful connection, not a serious character judgment. Whether it is praise or teasing depends on both the name and what the person did.
3. Examples
“닉네임이 정리왕인데 표를 깔끔하게 만들었네. 닉값한다.”
Your name is Organizing Champion, and you made a neat table. Living up to the name.
“매번 늦던 사람이 오늘 일찍 와서 지각대장 닉값을 못 했어.”
Our usual latecomer arrived early today and didn't live up to the Late Champion nickname.
Sources
References support the meaning and usage. Example sentences are written for NoCapWiki.