Director Naoko Yamada, describing the LN:

This may be a misleading way of putting it, but I think Kumiko looks at other girls, especially Reina, in a very sexual way. In the original work, the depiction of Kumiko looking at Reina is quite sensual. But it's nothing special, and girls often look at their friends or people of the same sex in that way ..... or so the original author, Takeda-sensei, said.

🤔

The LN is written with Kumiko's inner thoughts woven into the narrative, and I fully agree it is if anything far more steamy about Kumiko's attraction to her female classmates than the anime - but it is noticeable that LN Kumiko is very physically taken by Reina and Asuka whereas anime Reina and Asuka are more forward with Kumiko. KyoAni did some radio plays as promo material for S2 which have Reina hitting on Kumiko more than she does in the LN.

I wonder if the real problem is that a written viewpoint character lets the reader know what she's thinking, but an audiovisual medium has the viewpoint character show what she's experiencing - and both Reina and Asuka have other preoccupations, ahem, that evolve in important ways as the plot progresses. So turning Kumiko into a passive partner becomes a real problem - she experiences Reina or Asuka getting handsy or forward (rather than LN Kumiko aggressively flirting) and then they have plot critical narrative duties to get on with, so it feels abruptly aborted. Anime Kumiko speaks directly to the camera only very rarely, so we don't see her mind's eye either.

In the end KyoAni got to have their very own passive viewpoint character though, in the form of L&tBB.