Does this even matter? I mean, if you really wanted to stop people with weird views from grouping and then getting extremist through evaporative cooling[0], you'd have to roll the world back to preindustrial times (and even then you wouldn't stop it completely). Mastodon is but one platform; fringe groups will happily keep relocating when their old meeting places want them no longer, or they'll even set up their own platforms. Now that physical proximity is no longer a limiting factor, people of like beliefs will always find each other (if they bother to look).
(I think a good analogy is this: a Mastodon community is in the same relation to the Internet as Facebook Groups are to Facebook, or subreddits are to Reddit - a federated system inside a larger whole. The way fringe groups use subreddits/Facebook groups is analogous to how they will use Mastodon.)
If you're worried about extremists getting endangering public safety, I feel it's the job of the police to infiltrate such groups and stop individuals as they cross from talking to doing. If you're worried about large chunk of population catching some insane beliefs; I offer no remedy for that.
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[0] - https://www.lesswrong.com/posts/ZQG9cwKbct2LtmL3p/evaporativ...
And honestly I'd rather have that than the current state of social media where everyone is on the same platform and yelling at each other and using retweets to shame one another. My experience is anecdotal of course but whenever I see people trying to engage in discourse with the other side, everyone just doubles down on their opinion and the conversation goes nowhere.
Depending on the world view that can be perceived as a bug or a feature.
As far as I'm aware, "being an accurate source of voting intention" is not a prime requirement for social networks, nor is "making life easier for lazy journalists/pollsters". At least, not for any where the users are the customers. For those, where the users are the product, requirements might be different. Personally, I view this is a positive for Mastodon.