Mastodon builds on the ActivityPub protocol with its own API, but the last time I looked into it, the documentation of the API was poor, so it was difficult to develop for.
I haven’t looked at the BlueSky docs so I can’t compare, but it sticks in my craw a bit seeing the words “friendly for third-party devs” being used in the same sentence with Mastodon
The ecosystem is not so healthy on the server side, and I think the API documentation is a major factor.
The project has been sitting on this for many years
You could say it’s an improvement over BlueSky because with Mastodon you can host your own server, but if you don’t like their server, then you’re out of options