Mastodon is extremely technologically stagnant, with one and only one player controlling the entire technical ecosystem.
There is a huge lack of interesting apps or innovation. The protocol is very narrowly defined, and does not have any Postel's Law characteristics. You have to use the limited API offered in the Mastodon form.
There is starting to be some interest from ActivityPub in defining their own client APIs. But this gathering together of people is only just starting, and it's unknown where that effort will go or when it will see traction in adoption.
It also sucks being moderated by whatever Fedi you are on. And the very poor state of account portability sucks.
There's also internally a very aggressive culture, against devs doing fun and interesting things with the social media data. The social rules of engagement seem to be that you can only enjoy Mastodon as a feed that goes by, and you cannot download or analyze deeply. You can't index or search, except with strong carved out cases. People building search or indexing tools are harassed and abused.
I strong recommend this excellent write up, which does a far better treatment than I could offer, on Mastodon: https://connectedplaces.online/reports/fr156-share-where/
Atproto has none of these issues, and has a rich ecosystem of devs building neat tools like RSS readers (skyboard), writing (greengale), book reading (bookhive), trail making (side trail), annotations (margin.at), events (smoke signal), chat (roomy), social/research bookmarking (semble), video streaming (Streamplace), media watching (pop feed), key attestation (keytrace), git (tangled), and yes, devs: containers (atcr).
I'm sorry but Mastodon is a dead end, a bad design, going no where, bereft of interesting dev engagement, with data that is hard to share & make interesting use of, data that is extremely narrowly constrained in shape/form.
I am envious of the dev-centric culture there. But it astounds me that devs would choose to exist on a place that is so technologically unalive & so low potential.