Hmmm... My one complaint about this (besides that its name conflicts in searches with the game by the same name)... has got to be those shoulderplates.
It is worse in the anime for sure... but even in this, the design of Momonga's default costume is atrocious. The enormous shoulderplates look like extensions of his skull. And given his arms are mostly obscured it makes him look like some sort of overly dramatised frame for a painting or a doorway. The sort of doorway teenage goth girls would dream of having for their bedroom. That or that he has the world's biggest metal ears.
At least his other outfits as they showed up in the anime were better. Hopefully they'll also be better here.