Believe in it?

Nothing to believe in, it’s a word that describes an evaluation of events on a subjective level.

Person does bad thing, bad thing happens, other people decide that the bad thing was good because it happened to the bad person.

Secondary to that, they believe that the bad person’s actions led to the bad thing happening to them.

Comeuppance isn’t the same thing as fate, karma, or doom, all of which do require abelief in external forces. It just means that people think any bad things that happened are appropriate