Why? Author can have Marin be a completely different character with no writing to do it over and over, have her not react to see the guy she loves in his worst state yet, can ignore him for free without it ever being acknowledged and just time skip through it and now misunderstand on her own that he is sacrificing what was formerly most important to him to make her cosplay and act like a mentally unstable bipolar weirdo.
the last 3 instances of "drama" were her randomly turning into a 5 year old child with speech impediment.
I'm less down on this chapter than many of the people in this comment section - while I don't particularly love the drama, I can see how it can help surface and resolve some of Gojo's issues and, assuming the mangaka is on the ball, bring the two main characters closer together - your comment did bring another consideration to mind. Marin's reaction to what happened is
deeply disheartening.
She's the love interest of the series, but any romance between her and Gojo is hampered by how cautious and cowardly she gets any time he's in emotional trouble. Even back at the first misunderstanding between them, when his grandfather was at the hospital, tests were coming up, and he had to man the store himself while making her first costume, nearly killing himself in the process - she just kept away, not trying to get closer to him to figure out why he wasn't responding to her any more. She never tried.
Of course someone can bring up excuses for her - she didn't know him that well, she had no reason to suspect he was that ride or die right off the bat, he should have just talked to her and said what was going on - but she's the extrovert in their relationship. The one who in every other interaction is more forward, outgoing, ambitious. She's a Manic Pixie Dream Girl, except when it comes to the actual part of that character that's beneficial to the (usually) morose, shy, self-absorbed protagonist.
And sure, she apologized. But apologies mean more in action than in words. Now, Gojo seems to be going through that same psychological difficulty, and instead of reaching out, instead of reassuring her friend and asking what she can do to help repay him for everything he's done for her, she retreats. Allows herself to wallow in self-pity.
I really hope that summary isn't accurate. I hope that next chapter, she finds her balls and actually fucking
talks to the guy she thinks she wronged. But there are a lot of plates spinning in this manga right now and frankly the odds of the mangaka bringing them all to a halt without some awful melodrama is pretty low.
Edit: I think that's a good summary of the main problem with Marin as a romantic interest actually; she doesn't actually provide any sort of emotional support to a partner who desperately, obviously needs it. The only time she says supportive things is when he provides
her with what
she needs. Their entire relationship is heavily skewed towards her and her desires. And that's pretty typical of romance stories, to be honest, but it's not a great model for a realistic or healthy relationship.