I would hope, like you say, those larger instances would naturally realize larger donations with the higher user volume.
But sure, each instance can make the choice to inject ads in their users’ feeds. Larger instances may have promos with companies like video game publishers or movie studios. But I think the natural infrastructure that the Fediverse provides, limits this from the crazy ad levels we see in today’s failing mainstream, centralized social media companies.
I switched instances just so I could be on one that had downvotes because I passionately believe downvotes give immediate power to of self moderation to the users. However, I’m very light on downvoting. I only downvotevote stuff like the OP was talking about or if something is technically incorrect and damaging to a thread. But again I’m very light on downvoting, as I was on Reddit before. I usually ignore something that I simply don’t agree with or think is too silly, etc.