MLem (the iOS Lemmy app) was also showing the user karma (but I think it was only showing karma gained on the local instance). So I guess this is nice for people that like to know their karma.
I also agree with @nlm that we should leave this as a thing for yourself. The Lemmy API should not bother with reporting user karma as It would be way too easy to cheat for people with singe person instances. (and of course the toxicity that comes with karma)