My Hero Academia Chapter 366, "Full Moon," injected some humor into the series' final battle. Mirio Togata showed his butt to Tomura Shigaraki as a temporary distraction. For all of the grim things that have been happening, this moment seems wildly out of place.
In fairness, it's not the first time a final battle in a shonen manga had such an odd moment of comedy. However, where those scenes may have worked, MHA's scene might still come off as a little too weird, especially given its context. It's okay to have comedy in a final battle, but it has to be properly set up and warranted.
To better understand this, here's what MHA Chapter 366 did. As Deku was arriving at UA, the barrier surrounding the campus needed to be taken down for two seconds so that he could join the fight. For the battling and badly beaten Heroes, keeping Shigaraki distracted for two seconds would be far too long.
Mirio's mind raced for a solution. In his desperation, he thought about what Sir Nighteye taught him about the value of humor and smiles. Thus, to distract Shigaraki, he rose out of the ground, presented his butt, and screamed "Full moon risin' tonight" at the top of his lungs. The confused Shigaraki left out a "pfft," which left just enough time for Deku to enter the barrier and slingshot himself into the villain. In this sense, the gag worked perfectly.
However, the real issue with this gag is its narrative context. For several chapters now, the Heroes have been engaged in a long, harsh, and desperate battle against the villains. Right now, Bakugō is clinically dead and Edgeshot is sacrificing himself to resuscitate him. Much of what's been going on has created a dour mood. Looking at it like this, adding comedy, no matter how good or how brief, should come off as unwarranted and forced.
This isn't to say that having comedy in a final battle is inappropriate or even that it's wrong for MHA to have some. It can be a welcome surprise as long as the narrative properly accommodates it. Plenty of MHA's predecessors in Weekly Shōnen Jump have accomplished precisely this.
A similarly brief moment of comedy came up in Naruto during the final battle with Kaguya. In Chapter 682, "Bet Ye Never Seen This," Naruto used his Sexy Reverse Harem Jutsu to distract the Mother of Chakra. It allowed him to successfully land a hit and nearly helped clinch the fight.
This moment of humor worked for a few reasons. for one thing, it allowed Naruto to showcase one of his oldest and most infamous techniques one more time before the series conclusion. It was also meant to rebuff Black Zetsu's claim that all of shinobi history followed his design and prove that it was shaped by the efforts of individuals. Furthermore, it's far from the only moment of humor introduced during the final battle; plenty of characters had funny things to say and do throughout the battle with Obito and Madara before this one. This meant that the general tone of the story at the time prevented the comedy from feeling too out of place; the characters, especially Naruto, were in relatively high spirits despite their dire straits. All of these things meshed together well enough to prevent the Reverse Harem Jutsu scene from feeling too forced or awkward.
In fairness, MHA Chapter 366 could be said to have done some of these things, too. Mirio showing his butt was an example of the sense of humor he developed while working with Sir Nighteye. Creating levity to counter the despair that villains bring isn't just a Nighteye lesson, either; Pro Heroes always try to keep spirits high to assure the people they're saving that things will be alright. There have been some moments of lighthearted humor, even if they're few and far between in this section of the story. In these ways, the Mirio joke can come off as more unexpected than unwarranted.
With that said, the lack of lighthearted humor does Mirio's joke feel somewhat awkward and out of place. The humor that's been shown in the war against the villains has often leaned closer to the dark and twisted. It's more like the kind of humor one would see in something like Jujutsu Kaisen or Chainsaw Man. Between that and the intense fighting, it's difficult to conceive of a time where light humor like Mirio's would be appropriate.
However, one could argue that this moment of levity is still in line with the rest of MHA's overarching narrative. It's only when the villains are present that things get especially dark and the characters need to get serious. The rest of the story is upbeat enough that the humor and hijinks don't feel out of place. Mirio presenting his butt Shigaraki is a reference to a past event with such hijinks. In these regards, the full moon joke fits both the overarching story and the character that's doing it.
What MHA has done with its story is the reverse of what happened to Dragon Ball Z. After many of DBZ's major story arcs were about intense, high-stakes action, the Buu Saga took a more comedic approach to things; it was like a return to form for Dragon Ball, though some audiences weren't happy with the change. Looking at MHA's joke like this, the Mirio joke could be seen as a return to form for the series.
However, there are better ways to balance comedy, action, and dark moments in a series. If MHA had taken more after something like DBZ or One Piece in these regards, it might have been able to make the Mirio joke land better. As things stand, the story is too dark for a joke like this to just pop up without raising a few readers' eyebrows.
In defense of the joke, it did receive plenty of buildup within the context of Chapter 366. Much of the chapter is about Mirio trying to think of a way he can make a difference now that Shigaraki knows not to focus on him. The mounting tension, the way he scrapes the recesses of his mind for answers, the way his face contorts in distress, and even the name of the chapter are all leading up to the moment where he moons the villain.
On top of this, it also has a resounding payoff. It can be looked back on as the move that allowed Deku to properly engage Shigaraki. Even Naruto's Reverse Harem Jutsu didn't have this profound of an effect on the fight with Kaguya. For better or worse, Mirio's joke ended up becoming a necessary and important plot point.
For how odd Mirio is for showing his sense of humor at this time, it isn't anything that breaks the story. It's in line with the character and the mood that's been established throughout much of the series. This instance of comedy in a final shonen battle can simply be chalked up to one of many. As long as Horikoshi doesn't get carried away with this kind of humor at this point in the series, the story should be able to end with its dignity still intact.