Leaving a sugar skull offering in The Sims 4 does not have any noticeable effect on the game. The game does not recognize or acknowledge the offering, and there are no associated events, interactions, or changes in gameplay.
Sugar skulls are a decorative item in the game, primarily used for aesthetic purposes and to add a festive touch to your Sims' homes, especially during the Day of the Dead celebrations.
While the game doesn't have any direct mechanics related to sugar skull offerings, you can still use them to create a visually appealing and culturally relevant setting for your Sims.