Yuuka is cosplaying as Kurama?

Considering the whole scene,and the fact that Kaguya is often portrayed as an otaku,yes.And the outfit is the same,but green.

I don't speak moonspeak but I do know of its habit of leaving the sentence's subject to be assumed. In the scene they are parodying Kaguya/Elder Toguro's line is "why wont you die!" Like Yukarin says, Kaguya is being shown illusions of a battle that will last eternity. I don't know if the relevant line is specific in the comic but it would be great if someone double checked it.

6163 said:
I don't speak moonspeak but I do know of its habit of leaving the sentence's subject to be assumed. In the scene they are parodying Kaguya/Elder Toguro's line is "why wont you die!" Like Yukarin says, Kaguya is being shown illusions of a battle that will last eternity. I don't know if the relevant line is specific in the comic but it would be great if someone double checked it.

Indeed, the subject of the sentence is entirely implied, not stated, so I had no idea how it differed from the original. Fixed now.

Yuuka is wearing Yoko Kuramas' outfit from Yu Yu Hakusho right? Either way definitely a reference to the Demon Tournament.