No. All you need is using ActivityPub plugin. Then who ever from (almost whole wide) Fediverse can follow your categories (and/or tags) if they know username of those so called actors — it is in similar form than email address, like @some-name@forum.example.com.
So you as an admin don’t need mastodon or any other fediverse account, because your forum is then one server in fediverse, and sends information thru ActivityPub if someone is asking it.
So you may want to be active in some Mastodon server and share your content. There is no algorithm forcing content to everyone. It is like email, or a mailing list — you must know someone’s address before you can send, or subcsribe to a list before you can receive anything.
As said, ActivityPub is a protocol how to deliver data thru Feviverse, nothing more.