curious, how does one go about "subscribing" to a person's feed via mastodon? Isn't the subscription input value normally someUser@someInstance.net (or https://someInstance.net/someUser)??
The full profile page itself also has the usual RSS Auto-Discovery META tags so that most feed readers will accept the profile page address itself and figure out the ATOM feed from that.
The only other twist would be for feed readers to add WebFinger support which would allow them to auto-discover the "full profile" page from the @some-user@some-instance.net style address. I don't think any feed readers have added that yet.
For example, copying
https://mastodon.social/@b_cavello (the URL for the main Mastodon artist)
will allow you to subscribe to either
https://mastodon.social/users/b_cavello.atom
or
https://mastodon.social/users/b_cavello.rss
in your reader.