That's something I like a lot about this manga. The characters are straightforward. Most other manga wouldn't tackle it like this. They'd dance around it for a while before taking action.
Pretty sure Maeda knows that's a stupid opinion to have of Ruri. And that's why she gets defensive when Ruri tries to get it out of her.
She's got a personal hangup, and she doesn't wanna say it out loud, because it's fucking dumb and she is at least somewhat aware of that.
Actively trying to figure out what needs to change so you can co-exist amicably isn't something someone who doesn't care does. Ruri is proving Maeda wrong simply by asking what's up, and like any person forced to confront being wrong about something, she's annoyed about it.
By trying to tell Ruri to ignore her, she's even trying to get Ruri to act in the uncaring way she expects. By verbally asking her to do so.
And Maeda's reason for helping just being "doing it for other people" might have her feeling all superior because Ruri isn't the type to fall on grenades for strangers... Except Ruri is right there with her, the other person from her class helping with the sports event.
She already knows she is gonna have to change her mind, and it has her pissed off.