This is because it’s emulating Reddit, and operates in the same ways, cultures, and countries as Reddit does.
From bad mods and bans, to niche communities, to the brigading and politics. From rating posts, avoiding discussions, and responding with meme floods.
Lemmy will merely reproduce all the negativity, politics, and everything else we hate about Reddit …and it’s already doing so.
[EDIT: As you can see in the comments, full of people agreeing Lemmy will also be toxic in similar ways].
The common denominator is people. Anonymity gives some people the feeling they can say things they would never say face to face, which isn’t all bad, but without filters discussions can be toxic. Been true since the early days, will always be true when there are few consequences for speaking without caring how it comes across.
I’m curious why you see niche communities as a bad thing. They were the shining part of Reddit, and many here complain that because of the decentralization it’s harder to get them to grow. Maybe that’s what you mean, they can’t get big? That’s not really a bad thing, small means more accountability potential.