If you know they're narcissists, then you should also know that narcissists are among the most insecure human beings on this planet, if not the most. They definitely don't like being made fun of, even if they mask it well.

https://www.nyu.edu/about/news-publications/news/2021/march/narcissism-driven-by-insecurity--not-grandiose-sense-of-self--ne.html