What I envision is a rating system that lets established accounts (like those with a minimum karma and public identification) up/down-vote a service, or tag a service with a topic ("General", "Techies", "Artists") or label ("Bigotry"), and vote on those tags (which would help avoid unfair labels being attached to a service), showing the tags only if they reach some threshold.
The tags would be curated by users with moderator-level karma, which itself can be based in part on the "accuracy" (agreeing votes) of your tagging activities.
It may even be beneficial to add a time window to the scoring system, so that only the past-so-many-months are considered. This can both eliminate bad actors (spam accounts to affect vote) as well as give a Fediverse member a chance to clean up its act and recover.
Meanwhile, client apps can simply support polling (and contributing to) the rating system, rather than sticking their necks out and getting rating-spammed by angry bigots ranting about "muh freedomz!" by maintaining a list of singled-out instances. The system is then opt-in and the data maintained by the community. This also means we don't have to rely on - ahem - a stubborn code owner to adopt the standard within Mastodon itself.
Overall, if it's thought out well and takes an effort to become a trusted contributor to the system (and the score is maintained by "most recent activity"), it should empower users to engage their own filters so the Fediverse can Fediverse and the client apps can simply ignore toots from low quality / filtered sources.
Mastodon x appdot.net = fun? A place for former ADN users - on the whole