I'm going to take the opposite approach and say Lemmy is surprisingly active.
I wanted to check an idea of Reddit proportion of lurkers; didn't find actual data and am definitely not emotionally invested enough to make my own, but it took me to the 1% rule, which states that on an online community, roughly 1% of people create content, 9% edit/update/upvote/comment, and 90% are passive consumers of content.
It's very true that Lemmy isn't very active (although it's definitely been better recently!), but given this, I honestly believe it's purely a matter of gaining a critical mass of users. So I see two main approaches:
- Comment as much as possible! Sometimes you'll be shouting at the clouds, sometimes you'll lead by example and people will join the conversation;
- As a community, let's get more people using Lemmy so that even with the same ratio, the absolute number of posters & commenters will grow.
I've seen the issue on Mastodon when I joined a few years ago, and my friends and I make a point of not using "favs" too much, just boosting (which puts it in your followers' timeline) and commenting/replying. It's a small thing but it's good etiquette and encourages others to do the same.