If someone isn't getting a lot of merit, it's not a sign that most or all of their posts are bad. Merit is a limited resource and typically it's awarded for outstanding posts and valuable insights. If you are making average posts, you might be not getting a lot of merit while not actually being a shitposter. Also, using total earned merit can show incorrect results, because sometimes there are posts that earn a lot of merit, dozens or even hundreds; so a user can have a large share of their merit coming from just a few posts, while the rest of their posts are average.