So as I understand it, Mastodon is a Ruby on Rails web server that speaks to other instances using the ActivityPub protocol in a technical sense.

You register with a given Mastodon instance which speaks ActivityPub to other servers which may or may not be Mastodon instances as well.

Now your home Mastodon instance also serves a React frontend which is technically acting as an OAuth2 client that speaks to the backend via REST APIs, compared to say: Server-side rendered HTML

Similarly, mobile apps connecting to your preferred instance are also OAuth2 clients.

In that sense, you can use a different frontend to speak directly to the REST APIs offered by your Mastodon instance of choice.

This all being one level above ActivityPub itself of course.