Mastodon for sure supports fetching individual posts over ActivityPub. For example:

    curl -L -H 'accept: application/activity+json' 'https://mastodon.social/users/Gargron/statuses/18614983'

It does have a bunch of stuff that isn't federated though, such as Like counts/collections. And of course it only implements the server-to-server (S2S) part of AP, not the client-to-server (C2S) part.