I feel like karma is mildly useful to identify potentially disruptive users, but it's so easy to abuse it that it isn't worth the trouble:

• you can farm karma posting low-effort but still generally liked content (I did this all the time in another site; it's damn easy). That negates any potential usage of karma to identify disruptive users.
• moderators start using karma as a low-hanging fruit way to discourage newbies from their communities.
• once you gamify the system, some people will game the game. You get for example mods removing content from users, just to repost it themselves for karma.
• accounts with a lot of karma become a commodity.

It's theoretically possible that someone creates an add-on that tracks your karma. That's fine - as long as it doesn't become the default experience, its impact on the overall Lemmyverse environment would be almost nothing.