Several things about the My Hero Academia cast have changed going into the final arc, especially their designs. Many of them have updated their outfits, added new support items, and fitted themselves with gear to deal with recent injuries. Some of them are even sporting new hairstyles.
More specifically, some of the girls have made their hair much more manageable. Mirko and Nejire Chan have gone from waist-length hair down to shoulder-length. Mt. Lady has either done the same to her hair or tied hers back; whatever she's done with it, it's no longer flowing down her back like it used to. Two instances of girls streamlining their hairstyles would be a coincidence, but three is a pattern. Maybe there are in-story reasons for this, or maybe Kohei Horikoshi simply got tired of drawing long-haired women. It's hard to say, but there are ways to figure it out.
Why Were the Girls' Hairstyles Changed?
The first heroine with a reason to shorten her hair is Nejire Chan. In the story, she had to shorten her hair because Dabi burned most of it in Chapter 292, "Threads of Hope." By the start of the next arc, she cut whatever long hair she had left so it was all shoulder-length. This is the in-universe explanation, but Horikoshi might be willing to share another story.
On Nejire's profile page, Horikoshi gave a few additional details about the Hero-in-training that weren't in the story. Among these details, he offhandedly mentioned how drawing long hair is hard. This may be the hidden reason he decided to write her hair out of the story.
Horikoshi may have targeted this point about drawing long hair towards Nejire, but that doesn't mean she's the only one it applies to. Long hair generally takes much longer to draw than shorter hair; it's also more likely to have strands that stray off depending on the person's position and motions. The more hair a character has, the more effort an artist has to put into drawing it. This difficulty is only compounded when there are more long-haired characters.
This would explain why Horikoshi took an opportunity to cut his burden loose. This would explain why he changed up not just Nejire's hair, but also Mirko and Mt. Lady's; whatever reasons they had for changing their hair weren't given by the story. Horikoshi might have seen this as another place to cut corners.
Of course, none of this is to say Horikoshi is wrong or even lazy for making this change. Plenty of Shōnen Jump artists have employed time-saving methods to keep up with the workload of drawing 14-20 manga pages every week; with Dragon Ball, Akira Toriyama gave Goku's Super Saiyan form lighter-colored hair so he could save time inking it in; with Naruto, Masashi Kishimoto gave Sasuke a cool new outfit, but had to change him back to his standard look due to unsustainability. If Horikoshi decided to change his characters' looks to something that would be easier to draw, that's his choice.
Whatever the girls' reason for changing their hair, it should undeniably make Horikoshi's job easier. Whether they did it because their cuticles got damaged in their last fight or simply to make fighting in general easier, they'll likely keep these looks for the rest of the series. It should be seen as a change for the better on everyone's part.
