I've been trying to find a good Marxist instance, but Lemmygrad and Hexbear are widely hated. Why is that?

Because the majority of Ledditors are either liberals or full blown reactionaries and the ones who are not range from some cringey techno-libertarian who think FOSS will usher in socialism to some radlib cruise-missile socialist who strangely never deviates from the US state department. The very few who could legitimately be considered socialists are some kind of anarchist. They certainly aren't Marxist.

Are there any good leftist instances?

Outside of those two, I won't hold my breath. The easiest litmus test in the world is how they're responding to the genocide at Gaza. There's very few places where you'll get complete support for the Palestinian liberation struggle and not whining about the imaginary babies Hamas allegedly beheaded or trying to equate the conduct of a genocidal nuclear power to a paramilitary fighting for national liberation.