Mufasa did not become a demigod. What’s happening is the ‘Fisher King’ trope, where the kingdom reflects its ruler. Mufasa was wise and respectful, and so the pride lands thrived. Scar was disorganized, petty, and focused on his own power, so the circle of life broke down. Simba rules like his father once did, so things return to normal. The rainstorm, washing away symbols of death and decay, start the process of restoring the ecosystem. That’s all.