It’s kinda just like, an avatar type deal, except the species is a closed circuit. As in the only limit is that any interraction outside the inggus universe is non-canon. And the only exception to that rule is Penn (see icon).

It’s both an “it’s my species” reason and that lore wise Penn is a space/trickster denizen. That’s kinda their thing, to break 4th walls and fuck with canon. Also Poppy really would shapeshifter into the character they’ve decked out cause they want to look like that. Iunno what to tell you!