Shimamura is, in a word, lazy. This is a trait constantly described and showcased in her throughout the story; she's passive, dislikes troubling herself, and avoids thinking too hard about things that bother her. A daydreamer, she describes herself as a navelgazer at one point, devoting much of her time to simply sitting around and thinking, losing herself in her own head and often getting sidetracked by all sorts of unrealistic, fantastical thoughts. There's a reason why she's described as a 'bit of an airhead'; Shimamura is full of fantasy and wonder, and though she's grown more misanthropic as she's aged, that remains.
Shimamura thinks about others and breaks down behavior and decisions in her head to try and understand it. Human relations trouble and stress her out, causing her to spend the little energy she has; she describes people as taking bits of her all for themselves, and expresses the fear that one day she'll lose everything of herself with how everyone around her steadily erodes her and sweeps her along.