Though it may seem like an insignificant detail, Jujutsu Kaisen is one of the few shonen series that allows its characters to bear the scars of their fights. While it isn't the first to do so, it's common for characters in similar manga/anime series to have some kind of ability that allows them to walk away unscathed. In Dragon Ball, for example, the Senzu Beans heal all injuries with little to no scaring. The only character to actually have a scar is Yamcha, and that particular injury occurred early in the series and off-screen.
Jujutsu Kaisen's protagonists have no such luck. After the Shibuya Incident, several characters, including lead hero Yuji, walk away with severe injuries, and some don't even walk away at all. Usually, there would be a member of the team who acts as the designated healer, a la Yukina from Yu Yu Hakusho. Not this time. Every character that sustained heavy bodily harm during this arc continues to show signs of the fight. There are sorcerers that can remove the effects of curses, but it appears that healing physical injuries is more difficult. The only real exception to this is Mahito, whose Idle Transfiguration technique allows him to reshape souls, thus healing any injuries. However, he's on the opposing side, so none of the other major characters are likely to ask him for help.
Yuji now carries a large scar between his eyes and a smaller one on the left corner of his mouth. He was also stabbed in the foot by Choso, which will most likely leave a scar as well. This is highly unusual in shonen manga, especially for the protagonist. Meanwhile, Ino Takuma was beaten mercilessly and sustained heavy internal damage from his fight with Ogami before being thrown off of Shibuya Tower. Luckily, Yuji and Megumi saved him. Several Kyoto and Tokyo students were frozen by Uraume, and that probably left a few of them with some frostbite. On top of all these injuries, several characters lost their lives during the fight.
Having so many characters walk away with heavy scarring is unusual in the shonen genre. Most of the time, the creator finds a way to heal such injuries quickly so that they don't have to remember to draw the new visible scars every time. If there is a significant injury, it usually comes in the form of something big like a lost limb -- such as Guts from Berserk losing his arm -- and even that wasn't permanent. That way, it's one less thing to draw rather than something new.
It's understandable why mangaka are hesitant to disfigure their characters. With their tight deadlines and some trying to manage multiple series at once, it can be difficult to try and remember who has what kind of scar where. Many series have a one-week turnaround, so artists need to work fast. Therefore, it's easier to create a healing factor or miraculous medicine rather than memorize a new feature. This also applies to animation, as it can be difficult for animation companies to ensure that a character is depicted with a new feature after so many episodes of drawing them a certain way. With the animation industry being so hectic, particularly in Japan, and most of the time severely understaffed and over-budget, making sure each character is on-model can be a daunting task.
While it's not completely unheard of for main characters to show the signs they've been in a past fight, it's highly unusual for multiple characters to have such a major change at one time. The Shibuya Incident isn't the first major event to cause such damage, but it is one of the most recent and one of the most impactful. Not only was Yuji dealt heavy blows and permanent injuries, but so were many of his colleagues and friends. If a character has a scar, it's usually one of their first defining features, or if they gain one in the middle of a series, it's usually a large, significant one rather than multiple smaller ones in several places. While not groundbreaking, Jujutsu Kaisen has set a new standard for battle series and how they handle permanent marks on their characters.
