We talked about it here a month ago when it was still in development, so I wanted to give y'all a heads-up:
Jetpack 11.9, scheduled to be released in a few hours, will include a Mastodon sharing button.
I talk about the feature and how it can be customized in this blog post:
Share your blog posts on Mastodon with Jetpack
Hopefully it can be useful for some of you!