Rather than dense, I think Shimamura's actually really perceptive. She picks up on a lot of stuff and understand most of the fine details of social interactions, but what actually might be the issue is whenever the behaviour Shimamura picks up are directed towards her. I think Shimamura is her own blind spot.
In general, she doesn't particularly perceive others nor herself as special, and just goes with the flow of these relationships being naturally created because the time and space allowed them to, and then the same relationships naturally fading away when people separate when they're no longer convenient.
Adachi and Taru don't fall into these categories of a natural flow, however.
She kinda gets that Taru and Adachi are gravitating towards her for some reason, but she doesn't know and wouldn't understand those reasons. I feel like she has a hard time pinpointing what her gut feelings and her intuition are telling her exactly because she doesn't relate to them, and finds those actions/desires strange. At best, she dissects the behaviour she observes and leaves them at that, without attributing a meaning to them if it's beyond her understanding of it.
The closest she can relate most of Adachi's actions to is her sister, and after meeting Adachi's mother, she's come to the conclusion that Adachi lacks affection and wants to be treated like a younger sister because she comes from an emotionally distant/broken home. Ever since she's settled on this explanation, I think it's further hindered her understanding of Adachi's actions, because she attributes most of her suspicious behaviour to this reason and generally falls back on it.
Other than that, when it escapes her understanding and she can't use any other experiences to explain it, in a very Shimamura-like fashion, she just accepts that Adachi/Taru are doing this and that, and just lets those things happen. Because even without understanding the specificities, it doesn't particularly bother her anyway, so there's no point in going against the flow.
"Well, whatever."
