Wigglytuff are known as a cute species of pokemon which exude happiness and whose song can brighten the darkest day. They are considered to be pokemon as cheerful as Cubone are sad. This is the impression they strive to give off, and most of them are quite skillful at it. It is also an absolute lie.
Few Wigglytuff are orphans, to be fair. But the moon stone warps something in every Jigglypuff's heart, filling it with memories of their bitter exile from the Moon and float through space down to earth, of long-lost friends who got hit by asteroids before they could finish the journey, of shunning (in the case of wild Wigglytuff) from their friends and family, for evolution was said to be forbidden until the Moon was theirs again.
Some say that when a Wigglytuff opens its mouth, there are no teeth and no tongue, but a darkness impervious to all light which spreads throughout the pokemon's body. This is in all likelihood a myth, like that of Wigglytuff being immune to psychic attacks and afraid of bugs, but the metaphor is an apt one.