There is an immediate flicker of indignant outrage at the first question (even though it was Rita who implied it first), but she doesn't get to say much more than "What!? No--" before he's already moving along to the next question.

As for why 9S is an idiot...

"Lots of reasons," she adds unhelpfully. "Like... that stuff we just talked about." As far as she's concerned, their conversation about the ball counts as discussing reasons why 9S is a big idiot. The misunderstanding was all his fault and had nothing to do with Rita getting caught up in the atmosphere or jumping to conclusions or anything that would make her even slightly responsible.