Since it IS short, after all.
But I am long-winded, so I apologize in advance for a critique that's probably almost as long as the mission. ^_^; Please know I'm not trying to lecture or scold, just to explain my impressions and offer advice I hope is helpful for your future writings.
This mission felt... detached. There's nothing seriously wrong with it, but it begins and ends without the agents seeming to experience much in between. Not that there's much fic TO experience, but even so, I feel like the mission could have been developed more.
For instance, since Y/N fics are basically You!fics, what if the first agent through the portal became Y/N and had to go through the events of the fic themself? That would provide an opportunity to show why a story in this format might fail to be the immersive experience it's supposed to be when it's this poorly written.
(Of course, the agents could also avoid that problem with a "You" Crash Dummy, or maybe a SimGen, if they planned ahead.)
Kaguya's Angry Mode also just... happens, without any effect on the course of the story that I can see. After the mission, Kaguya's mood goes right back to where it started. If you wrote it without Angry Mode, you could remove the strange stylings around his dialogue and make for an easier reading experience, and the story could still end the same way.
Things that happen in a story have to have a reason for happening. The stakes don't have to be huge, but there should always be a sense that things could have gone differently if the characters had made different choices. Here, neither of the agents choose anything. Momo wishes she could cheer Kaguya up (perfectly good stakes!), but then a mission comes along, and she just goes along with it, like a cork in a river. She takes no action to try to turn the situation to her (and Kaguya's) advantage. I agree she didn't do anything wrong, because she didn't do anything at all. Thus, I don't feel any sympathy with her remorse at the end. Like the rest of the story, it just... is.
On a technical note, may I introduce you to the past perfect tense? Most of the two paragraphs about the Pennacook Club interview should be written in past perfect, since you're describing events that concluded before the time of the story. Mostly, switching from past to past perfect involves writing the relevant verbs with "had" in front of them, like so:
That morning, Kaguya had gone to give his interview for a coveted membership to the Pennacook Club. To this end, he had donned his best "gentleman novelist" outfit, had spent a bit more time on makeup, and had not forgotten to quote Tanizaki Jun'ichirō whenever he could. The interview had gone smoothly… until the end.
It seemed as unbelievable as it could get. It was not without reason that a member of the exclusive club had thought him worthy of an invitation: cultured botchan, tea ceremonist, "standing like a shakuyaku, sitting like a peony, walking like a lily." Yet, he had failed; he had been immediately judged unfit for membership at the interview stage.
(Also with miscellaneous other P&G edits. And, btw, did the interview go smoothly until the end [p. 1], or was he immediately judged unfit [p. 2]? Can't be both!)
Also, from the next paragraph:
> Was it because he had studied the Urasenke school of tea ceremony
> what had happened, happened
(Personally, I'm guessing the interviewers just figured he was too uptight and no fun at parties. {= P )
~Neshomeh, not angry.