I think Mastodon has the tendency to grow into the opposite extreme of toxicity: fragility. People as well as instances are very generously blocked, leading to perfectly peaceful bubbles.
Which are not that great. There's almost perfect ideological agreement...but not really. A handful of people dominate conversation by posting frequently whilst the rest barely engages and/or is afraid to speak their mind.
As an example, on a fairly large Mastodon instance I saw a popular user claiming how we should still wear masks. There were a few dozen replies, all in perfect agreement.
That's truly bizarre, as this is a topic that people have strong opinions on across the entire political landscape. Even amidst just progressives, there's no uniform consensus, granted people are free to speak their minds without repercussions. The fact that there wasn't even a hint of disagreement or nuance I find telling.
So I agree with your point, this too is a type of toxicity, just a different flavor. Fragility, toxic positivity, I'm not sure what to call it, but it's not healthy.
Another fun opposite effect (in comparison to Twitter) to reason about is algorithms and amplification. Mastodon has very little of that which is considered good. It's a more "organic" social network.
Quite a few users will discover though that an organic social network makes the chance of getting engagement on your posts even harder than it already was. Just getting your post seen at all is a challenge, and building somewhat of a following can takes months if not years of purposeful effort. This means that for the typical user, the feeling that you're screaming into the void will be common, leaving the question: why post at all?