Quote Originally Posted by Slashley View Post
Yes.No.

Think of it as a element-specific C frame Def Down. It'll get you to -50% quite easily, and other elements won't get it, but otherwise it's "just" another Def Down.
In addition, the wiki page for light Tsukuyomi points out that the element resist debuff also reduces resistance against debuffs from that specific element. So light resist debuff functions both as light-specific def down and a debuff resist down effect for light-only.
So if you don't already have a normal debuff resist debuff, you use the element resist debuff first, then try to throw your other debuffs (from kamihime of the same element).