I think if a instance owner decides to try to profit, it can happen. They could let advertisers have an account that promotes products and allow such posts to bypass community rules and disregard vote counts. You could theoretically profit from running a lemmy instance. But now your instance risks defederation and user might start leaving.
Edit: I think the smarter thing could be just asking for more donations than is needed to run the server, and pocket the excess funds. That could go on undetected as long as you falsify your operating costs to make it seem as if more funding is required. I mean I don’t know if someone could actually make a living of asking for absurd amounts of donations.