Never like Mastodon (neither did I like Twitter, X or whatever tf it is now) because of the no-community nature of it. Therefore, can't speak for Mastodon vs Lemmy.
However, I do see the "why so serious" problem on Lemmy a lot compared to Reddit. It can be depressing at times honestly. Lemmy really needs to up its shitposting game. Unfortunately, I do find myself going to Reddit when I want to come across uplifting posts. I really don't want to know how the world is inching towards fascism every day at such times.