Most firms do not treat the case and fit portions as one blended impression. Interviewers usually form judgments across several dimensions, so a weak answer can hurt without automatically erasing a strong case.

What matters is how weak it was. Rambling once and then recovering is different from showing no clear motivation, poor self-awareness, or consistently weak communication across the fit portion.

I would not try to score the interview yourself afterward. If there is another round, use the experience to tighten your stories and answer the question first before adding detail. If useful, this guide on case interview rounds and structure gives more detail.

tbh I had one fit answer go completely sideways and still moved on. I think the pattern matters more than one imperfect story.