Any Lemmy/fediverse instance could come up with a localized monetization scheme for people that browse through it, but it wouldn't affect other instances (or if they were injecting ads into feeds, they'll just get blocked by everyone else), but for the most part, it's got more of an IRC server vibe, no monetization needed when community volunteers are plentiful and the barrier to entry is low. Eventually 'big boys' like Lemmy.world will want a more formal and reliable way of paying for their server and bandwidth needs beyond primarily unsolicited donations ($ and time) by volunteers.
These are not profit generating services, they are community services. For now.