Adding a link to "mastodon.something" seems to register as a link to mastodon, but mastodon instances (servers or communities, if you wish) are much more diverse than that!

I'm on one which URL is indieweb.social for instance. Which means if you filter your "Who to follow" list by "Linked Mastodon Profile", I will not appear in it although it's a legit mastodon profile.

Ideally adding a link to a mastodon profile should work similarly to https://joinmastodon.org/servers but I get it may be complicated to implement.
Why not let us mark that link as a mastodon community, then?

As in:

1. Add link
2. Input link
3. [NEW] Checkbox "This is a Mastodon profile"
4. Save