Chances are that Lemmy has about the same makeup of people as Reddit, mostly. I'd say that where the difference lies is in the amount of tolerance they have for Reddit shenanigans.
Stupidity on the internet is amplified with resonance. One says something stupid, trolls or being rude, the other responds and the intelectual level of the discussion is plummeting. So my theory at least is that there's about the same percentage of "stupid". We just haven't yet reached critical mass.