If there's someone out there who likes Mastodon and has a bit more artistic talent than I possess …
You totally ought to make a https://shields.io/ badge for Mastodon. I see their "follow me on Twitter" badges on all sorts of GitHub repos, and it'd be great to have a Mastodon badge too! And recruiting people who browse code repos seems like a nice way to build the community.
You could even use the logos in the Mastodon press kit https://joinmastodon.org/ page (at the bottom)