Except the only "evil" thing that Mueller did was investigate Trump after he bragged about firing James Comey for investigating a potential Russian interference in the 2016 election.

Moreover, I'm not sure it is right for the president to celebrate this; isn't part of the job of a president to be diplomatic? It's one thing when a nobody like me celebrates the death of someone bad, but I'm not the president, no one expects me to be diplomatic, and generally speaking no one actually cares what I think about anything.