I think Yuzumori's reaction might have something to do with a past incident. Something similar might have been said but she got betrayed in the end, hence the disgust/fear at Mimika's words.
I'd say it depends on the exact construction in Japanese. If it's along the lines of "I don't want to be friends and nothing more", a misunderstanding where she stopped listening after "friends" would be likely. There's one other possibility I can think of, but to best explain it, I need a timeline:
- All three go to a park
- Mimika leaves, Shiroi and Yuzumori "talk"
- Mimika returns, Shiori leaves
- Yuzumori confronts Mimika about the differences in how she treats her and Shiori
- Mimika asserts Yuzumori is special to her, despite Shiori's best friend status
- Yuzumori delivers her ultimatum and 'confesses'
- Mimika responds in kind, but then breaks down
- Mimika says she "can't just stay friends" while crying
- Yuzumori becomes angry then disgusted with Mimika
Following the events, I think it's possible that Yuzumori thinks Mimika's dumping her, possibly on the command of Shiori. Not helping is that "it's impossible for me to give up" doesn't necessarily flow from "I like you" and could be interpreted as referring to her relationship with Shiori - ie, she's not able to let go of her current girlfriend and has effectively rendered Yuzumori the other woman, with a tenuous, rather than committed, relationship. If her father cheated on her mother(or vice versa), her chances of interpreting things in terms of infidelity would skyrocket.
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