Well you have to remember that in her mind they're like family and so it isn't weird to share a room together. She views him as a brother (which is her being normally because this happens all the time irl. I think this is realistic, not far fetched and ditzy). Asking was actually very respectful because she didn't want to upset her partner (knowing he doesn't like Momo) and she didn't want to make him insecure.
I personally think it was a good move by Iroha. Kusakabe understands that Momo is Iroha's childhood friend and that there's no getting rid of/avoiding him, and Iroha knows Kusakabe is insecure, so in order to ease his anxiety she kept him updated on the situation. Plus, she's oblivious to Momo's feelings (as many are) and Kusakabe is aware of them. It's better to ask rather than assume because it could cause an argument later. And there's also the possibility Kuskabe would've been okay with it (but we know that will never happen).
At least they are honest to each other all the time, even to the point that they seem dumb or overstepping some boundaries. Unlike 90% of shoujos where tone of the couple always end up misunderstood and followed by irritating dramas etc.
Im dropping she must like to be controlled or something because his love is just starting to become annoying but like damn she had to ask him wow doesn't she have a brain of her own or is this girl just dumb from love like she left her brain behind or something.