TLDR/Summary: Boring, milquetoast in every respect, and completely derivative, Tsuki ga Kirei manages to do nothing but tell the most basic story with bland characters. It fails to have any depth in any facet as well as lacking any shallow, surface-level appeal, making this show uninteresting and unable to set itself apart. The glacial pace of scenes and bad character design only exacerbates theses problems. At its best, this show is plain white bread. At its worst, its a stale loaf without having any drinking water.
The problem with this anime isn't necessarily that there are problems (which there certainly are), but more that it lacks any strengths. In wracking my brain for positives, I could only come up with two things that I would actually call good: the characters typically acted realistically and the kiss scenes were done pretty well. That's it. You could maybe also argue the music, but I ultimately found it mediocre.
Now that I've gone over all of the positives, its time to delve into where this show really fails. Many of these failures could possibly be overlooked if the show was charming in other ways, but it failed to do anything spectacularly well to look past these shortcomings.
Firstly, none of the side characters had any personality. They were all very bland and had no input on any of the outcomes in the story (with the possible exception of Koutaro borrowing the phone or him getting jealous). None of their actions really mattered and none of them developed or even showed character traits. The whole point of realism as a genre is the characters and if you fail here in any form of media aiming for a realist portrayal your work will be boring. To be fair, this would be fine if your main couple can pull the personality of the show by themselves, but they definitely can't.
Second, the plot was predictable, shallow, and uninteresting. If you are going to do a plot that has been done a trillion times before, the only way to make it worthwhile is to deviate somewhere, whether that is concept, character interactions, or even quality. Tsuki ga Kirei doesn't. To call it milquetoast would be putting it lightly. It didn't innovate with its plot points, characters, interaction, setting, or even directing/portrayal. It was a cookie cutter plot, cookie cutter characters, etc. These can be fine each on their own, but if there is no unique aspects anywhere it creates a feeling that the story is not one worth telling. For the same reason I would not tell a story about how I went to the store and came back with a carton of milk, Tsuki ga Kirei is a bad story where very little exceptional, interesting, or unpredictable things happen.
Third, the pacing was horrible. I watched the majority of the show in 2x speed, and it still felt like characters were talking slowly and scenes were dragging on. I actually enjoyed the show more like a 4.5 than a 3, but the fact that it felt only normal in 2x speed was certainly a knock. Additionally, the show took 3 full episodes to stop being complete garbage which is too many when you only have 12 episodes.
Fourth, the character designs (specifically the main). Most of the faces were the exact same. Ultimately, this isn't actually that big of an issue, but it was exacerbated by making the two main characters look almost exactly alike. This admittedly may not be a problem for everyone, but it is a huge issue to me when we are supposed to see these characters as unique.
Fifth, a lack of development or depth to the main characters. This also extends to the side characters, but is clearly of more importance with the main cast. They never change in any way or show any moments of human complexity. To be fair, change isn't absolutely necessary if you make a point of resistance to change or create a change in viewer perception of a character, but none of this happens. Even if a character never changes, if their complexity as characters is displayed that is often enough to be engaging, but they were utterly bland and simple characters. To be fair, you could argue the characters different displays of emotions (predominately jealousy, embarassment, or fear of rejection) were these complexities and in a way you would be right. The only problem is they were just standard reactions, with no more complexity or depth than if an author simply wrote "He felt jealous." It was the equivalent to the tsundere heroine meme that is "It's not like I care about you or anything, baka." Its just basic and uninteresting unless there is more going on or some context, which there isn't.
Additionally, I had some pet peeves that weren't drastic errors, but should be noted. The credit ending was sweet and endearing, but didn't fit the tone of the show imo, Akane's actions to me are too diabolical to be realistic unless they painted her as an evil character, the story he wrote at the end being discovered is a little too convenient, and the fact that they never brought up his interest in boxing and the Ali poster was a huge missed opportunity to actually add something to the character.
I don't understand how anyone can give this show a 7. That might sound arrogant, but it is undeniable that it wasn't innovative, it was purely basic. If you like basic thats fine, but calling anything purely basic more than just good seems intellectually dishonest. As I said in the TLDR, at its best this show is plain white bread. I can understand how someone can like plain white bread, but rating it 8 or high baffles me. At its worst, its stale, tasteless, and drags on forever.