Rin, Utsuho, Satori and Koishi are from former hell.
Eiki resident mostly in hell.
Byakuren was sealed away in Makai that is somewhat similiar to hell.
None of these is DARK. So still, Clownpiece should not be DARK because... this type is for characters that are clearly evil and malicious in nature, like Remilia and Mima, or characters that have darkness or black colour in theme, like Rumia and Lilly Black.
Clownpiece is neither evil or black. She's just resident of hell that is also fun-loving fairy, with pretty bright personality and costume. Not every fairy is Nature-typed, and it does not work with her battle style, so nope.
Fire/Heart or Fire/Reason are only valid options. Dark is not because Clownpiece is not evil/black. And Nature is not because Clownpiece is not type of character that would shoot leaves/vines/sunlight/sounds of nature on her enemies.