I think it's partially attributable to Lemmy phone apps haven't taken off yet. Phone posting is a different medium than desktop posting with regards to McLuhans "the medium is the message" principle. Phone usage appeals to the lowest common denominator. People use it as a time waste to mindlessly scroll. The cognitive load is much lower. Thus shitty content bubbles to the top because that's all the brain power people are giving it. Phone usage is not conducive to consuming depth of content nor contributing it.
Additionally I think the lower cognitive engagement tends to lead people into greater fuckwad behavior. You aren't on a board with human beings on the other side of a keyboard. It's just a stupid app on your phone.
I think if I'm right then in time Lemmy with death spiral not unlike reddit if phone apps mature and Lemmy apps become a mainstream daily use type of affair. I doubt that will happen without the capitalist engine driving it though.