I don't think Futaba is a fan favorite like Yumi, tbh. Yumi has always been popular from what I recall, so, like in real life, if you're popular enough, you can get away with anything.
I think this is a rather interesting topic. Because I feel like it's more of a "problem" (or maybe simply the intent) with the way the story is written. Futaba is just not a compelling character, as it has been pointed out by many people here, as well as herself and Akira. For instance, Io shares some traits with Futaba in her extreme lack of self-worth, yet Io is highly compelling.
Yumi bullies Futaba many times over, but that's just in the beginning which is then followed by "The Town has Two Halves" which radically changes our perception of her forever. I don't think it would be a stretch to say that we all feel some sort of collective guilt or pity for what has been done to her.
Then what happens? Yumi steadily grows, shows her affection for Akira at increasing rates, has a compelling emotional turmoil about liking a person that forced himself on her, has to endure being cucked by Chika constantly, either by being present or having to take care of Chinami for her best friend to have sex with the man she likes, gets in touch with her mom, gets a job, gets into the apocalypse squad, and not to count saving Akira so many times.
What about Futaba? (...) Her parents are architects and she writes an interesting horror story. Other than that, there's the stuff about Nodoka writing her book, but we already had Ayane's list of people that Akira was sleeping with, and her being assaulted by Kirin, which both served a similar purpose, way before this and were way more compelling too. Futaba just always feels to me like too little and too late. She could have known about what Nodoka did "in her name" and had something interesting happen out of that, but we were denied that too.
In fact there's so little about her that being bullied by Yumi is one of her defining traits and that hasn't happened since chapter 1.
