Most people hyperfocus on the 'what' and not the 'who' and 'why' in an arc like this. is it filler? you bet your ass it is. But in this story, we have seen that all filler is borderline significant.
So yeah, Iruma spent an entire arc crossdressing to help his friend and that seems irrelevant. but none of this is going to go to waste.
Gyari might be obsessed with Kuromu, but for all her overt declarations of love, Kuromu simply says "i know." why? because its not about sexual interest for Kuromu, its about utterly dominating the competition and leaving no room for doubt that she is the best. Gyari is so used to casually dominating others, that kuromu not bending over backward for her makes her seek a position of power to force a victory via submission. Kuromu is using her as a whetstone to sharpen her skillset. she literally told Iruma that she purposely is baiting Gyari to keep a worthy rival around because that keeps her from getting complacent and makes her stay on her A game.
Honestly, Gyari seems the type to join the back to basics group, which would make for an interesting adversary; one who can manipulate the emotions of others and make them want more.
Iruma is learning how a demons desire and obsession translate into something as complex as the entertainment industry. Akudols or Evidols, whatever the translation is, exist as a widespread and intentional emotional manipulation to keep the Evil Cycle at bay. Considering he is going against an organization that wants the Evil Cycle as the default state of being once again, I would think what he has learned here will be of some use.