Even if a platform tends to encourage meanness, stupidity, and radicalization, the specific direction that takes can depend more on founder effects. I don't know of any interaction design that made Tumblr in particular so radically left-wing.
It's also easier for a nice community to become mean than for a mean community to become nice, so maybe give Mastodon some time and exposure and see what happens.
Do you think it actually promotes radicalization, though, or is that just the culture that developed there? E.g. Mastodon's interaction design is very similar to Twitter's, but the people are waaaay nicer, because that's just the prevailing culture. Compare to other parts of the Fediverse, such as GNU Social, which is much more Twitter-like in tone.
Either way, I think it's a good lesson in how public-by-default social media is kind of a terrible idea in retrospect. You wanted to share a non-sensitive post, but maybe just to people who are in any way socially accountable to you, not the entire world.