There's a fundamental fact that people fail (or refuse) to grasp about decentralized services like Mastodon (and Lemmy) - they don't actually exist in any standard sense.
"Mastodon" is really just a handy collective name for the ad hoc community of innumerable individual sites that each independently run the Mastodon software.
And yes, as Rochko notes, there is nobody who can decide for the fediverse to block Mississippi (or for the fediverse to do or not do any other thing). It's necessarily up to the individual instance owners to do as they see fit. And that's very much the point.
It astonishes me how many people can't seem to wrap their heads around that simple but crucial fact.