Earlier quoted context omitted.
Depends on how carefully selected and considered your criterion for success is. It accomplished what it set out to do, which was not "make obscene piles of money" but since they didn't make obscene piles of money it probably is a failure anyway, right?
A peak of 2.6 million active users 4 years ago and the decline since then to 750k users is a true success story, right?
They didn't set out with the goal of making gobs of money or subsuming the entire online population, and because those things didn't happen, success is tautologically ruled out, right? After all, as we all know, the only kind of success that is even theoretically possible is being the best and only player left alive in your field. Literally anything less is failure, unqualified and unequivocal.
Mastodon did what they set out to do and there's a vibrant community around it. That's a success, even if it doesn't meet narrow-minded silicon valley definitions of success.