A relevant and good comment, even and especially if it opposes the opinion of the majority. Giving downvotes to signal disagreement, when posts are sorted by karma and very low karma posts are even hidden, leads to circle jerking and immediately kills every healthy debate and controversy in the bud. If I have a dissenting opinion, I want to argue, not be muzzled.

Boost for Lemmy?

Problem with a karma system, especially as it is handled over on Reddit, is that it will stifle dissent and promote circle jerking. You can vote controversial opinions out of sight even if they are totally valid but simply run contrary to popular opinions. If Lemmy got a karma system, it would have to work differently and allow for a healthy discourse.