Some form of aggregation may still be possible. Be it user by user, or server by server.
But like I said in another comment, for the fediverse to work the way you imagined, the total number of people doing content moderation would have to be orders of magnitude greater than even facebook's or twitter's.
Additionally the way it works is not mutually exclusive with differing ideas, only, in the way it actually works, instances that agree on moderation policy, can pool their efforts. Only where there are differences, are different communities and different moderators, needed.