I really have been scratching my head over what Jack Dorsey doesn't like about Mastodon. It's pretty much what he seems to want: a decentralized Twitter. The fact that he hasn't commented publicly about it seems to me that what he really wants is a "decentralized" Twitter...that he controls.
Mastodon is open source. I may be wrong but it looks like this platform is not. There's probably money involved somehow, which is why it is getting the Jack Dorsey bump.