Edit: And I've noticed the downvotes, yet what I have said is still accurate. It is true that Gab moved to mastodon, for example, and there is evidence of their mindset spreading to other properties in the Fediverse.
Some of the early adopters are definitely unsavory, yet that content is what new users are going to see. Peertube/etc are still too small to protect against it, I fear.
Early online worlds were (often, not always), academics and techies. Even that was often hard going to generate larger appeal. Facebook was Literally Harvard.
Random malcontents have an even higher social hill to climb.
An elite founding cohort tends to regress to the mean. An unsavory cohort ... tends to further radicalise or collapse entirely.
Fandom/hobby groups are more sustainable and often avoid pathological evolution, but rarely seem to develop mass appeal.
And all this before adding financial or business pressures.
If you seriously believe that the Earth is flat or that lizard people control the world, then frankly a Youtube disclaimer won't really change anything.
But take this random video I found earlier on PeerTube: https://peer.tube/videos/watch/7b5faff9-1752-4eba-be37-beeb0...
It's in French but basically it's one of these videos against the banking system that simplify and caricature the situation to the point of meaninglessness. Coming up with a proper fact-check for something like that would be incredibly difficult, and I'm sure extremely controversial.
It would also take a lot of time and money to get economists and physicists to weigh in on these videos. That's how these conspiracy theories manage to remain relevant in the first place: overwhelm the opposition with thousands upon thousands of hours of content, and claim that they take things out of context or that they don't get it if they don't bother to sink months of their lives watching all that garbage.