This way of developing the #fediverse where each server only receive one kind of the objects accepted by #ActivityPub makes it more fragmented it, right? A server should send and receive all kinds of objects and should be up to the client to how to processes those objects.
If an user wants an Instagram-like app just create an account on any service and use and app with that UI, of lager they wanted to see more kinds of objects they should just use another client that supports Note, Article, etc. with the same account on the same server.
Ideally all server should have a shared API.
This fixes #fragmentation, the need to have multiple accounts if you are into multiple kinds of objects/content.
One (small) issue is that it assumes there *is* a client outside of the server. Most server apps have both a web UI and an API, and it's even possible to build an ActivityPub server with no client API at all (microblog.pub was like that, IIRC). I think the ActivityPub C2S API was intended for servers that worked just like you're describing, but IDK if anyone ever really implemented it properly.