The sim becomes a skeleton by a buff, not a costume. I think the buff lasts for a whole day and then they will revert back to their normal self.
Carl Sims goes into which relic will cause which, but go to where they talk about the Balampalsoh (Cat) Relics. and then look for the one called Balampalsoh Death Relic - Blessing of Skeletal Transmogrification.
https://www.carls-sims-4-guide.com/gamepacks/jungleadventure/relics.php
This link goes into even more detail with illustrations, go down till you see Relics and Refined Crystals.
https://simsvip.com/sims-4-jungle-adventure-guide/#Skeletons
Once you assemble the correct relic you'll have the ability to turn your sims into living skeletons without cheats or mods.
