It actually seems to be calculated internally but only the instance host can see it, i think it is there to help with moderation or some other technical reason
Interestingly, the internally calculated value (which can be fetched using the API) differs quite a lot from the actual value that you get when you sum up the scores of all posts and comments. For example, my comment score is ~20x as high as what the API reports.
I made a tampermonkey script that calculates and displays your "karma" next to your username: https://feddit.de/post/1185964
(It only works for your own karma, doesn't show anyone else's karma, and it also only shows it to you. So no karma farming encouraged.)
I made it even worse, here's a tampermonkey script I made, that automatically calculates and displays your karma next to your username: https://feddit.de/post/1185964