I check out reddit every few days and tbh lemmy has the same amount of mainstream content. The only difference is that reddit niche subs are more active.
I think the fact that anyone can make the same community on a new instance diminishes niche communities more. If I pick a game on reddit ill find 2 or so instances with lots of use. On lemmy there will be 10 communities all mostly abandoned.
You can feel a difference on reddit though. Quality content and content numbers are greatly reduced from even a month ago.