I'm guessing that ads/sponsored posts would probably take the form of companies setting up their own Lemmy instances and pays to federate with other instances, and my could-be-unpopular opinion is that it's fine as long as it's fair, since if they have to abide by upvotes/downvotes as everybody else it forces the companies to make good contents, and that if they get too annoying/spammy they'll just get defederated or people will vote with their virtual feet and go elsewhere.
And the good thing about this approach is that you can make sure you get real interactions with real people at these companies for tech support, otherwise the ads would be completely useless.