Since this is a mod pack for Ultimate, you can do various grunts, attacks and taunts voices for Jigglypuff. You can create custom lines in your own with freedom, but it must be connected to the character's canon personality. Audio must be loud for the game to pick up. You may not add your own reverb and delay for the Star KOs and the Stamina KOs, it is coded in the game itself.

If you need audio references, here's one for Jigglypuff in Ultimate:

https://www.youtube.com/watch?v=nyrWAHlqrOE