I don't mean as of now, in Skyrim terms. Even when you were wandering during your time in Oblivion, could some NPCs have been dead? If a person pledges themself to a Prince, they are claimed upon death and are sent to their realm. So if Sheogorath claimed any souls, could they have been NPCs we have seen? We don't know how exactly everyone got into the Shivering Isles so it could be a possibility, yes?

Also, is the Hill of Suicide a thing invented by Sheogorath so people don't escape him by suicide? Do people in the domain of their prince truly die if they are in their realm? I know if a Mythic Dawn agent is killed in Tamriel, he'd go to Paradise because that is where followers of Dagon go. If a scamp aligned to Clavicus Vile dies in Vile's realm, doesn't that scamp die permanently? I heard they only go through the waters of Oblivion if they don't die in their own turf, unless they are a more powerful Daedra or Daedric agent such as Umaril. IF they die permanently, then that means anyone could escape by suicide, so Sheogorath invented the Hill so they can't escape until their natural death.

Thanks for the clarification, lorehounds!(Can I call you that?)

The people in Sheogorath's realm aren't Daedra, though.

And while I'd like to say not all of them are dead, some of them certainly live a very long time. A Bosmer named Dervenin was alive in both Shivering Isles and Skyrim - 200 years apart. Of course he's a Mer, so that could explain his longevity, and also gives a hint at the lifespan of Bosmer.

Daedra don't ever die all the way, so I see no reason why a Scamp who dies in the wrong realm would be an exception.

Well time in the Shivering Isles is slower than that of Nirns. So in theory many of the non Mer races who lived there could still be alive. But if that wasn't the case still many of the Mer living there would probally still be alive like Dervenin whom Clodius mentioned.

Wait, Dervenin was in the Shivering Isles????? No way! That is news to me!

I know the people in the SI aren't Daedra, but some could have pledged themselves or been conquered by Sheogorath, therefore making themselves subject to the SI.

Well you could think of it like this, each mortal who becomes insane and lives in the isles becomes part of Sheogoraths mind so in essence the mortals are then reborn as beings of his subconcious running around the isles, which in essence would make them immortal.

I don't necessarily mean are they still alive, but someone who lived on Nirn who lost their minds to Sheogorath and became his servant, would they go to the Shivering Isles and start life there? I seriously doubt everyone who goes to the Isles does so through all the portals. If they did, that means they were all adventurers and the opinion of adventurers is quite low.

He was High Priest of Mania. He looked younger then and unbearded, but he was there.

Sheo's new habit of being everywhere but not the isles has done a number on em.

Good point, in lore terms not gameplay terms there are probally thousands of people living in the isles and not all of them would have come through a portal. I'd say its similar to the likes of Nocturnal and Hircine, the followers of Nocturnal go to Evergloom when they die. And the followers of Hircine whom are werebeasts mostly go to his Hunting grounds to hunt for eternity. So when someone on Nirn becomes crazy they then belong to Sheogorath and when they die they are thrown into the shivering isles. I am not sure but my opinion could be true.

Can the "mortal" denizens of the Isles die a natural death? We know of at least one mortal from before the last Greymarch, making them around 1000 years old. Of course, that may just be Sheogorath keeping them around for their heretics.
What happens to the mortals who do manage to die in a non-suicidal fashion? One guy asks you to kill him so he doesn't end up like the people on the hill, implying something significantly different happens to those that die "naturally". I would presume that they just become a part of the Isles, since they are already Sheogoraths.
Edit:
I also am under the impression that nearly all of the mortals have died and were collected by Sheogorath. Or maybe they were just collected.

Two of the ingame books added by the Shivering Isles DLC has Sheogorath claiming souls after they die.