Wouldn't it make sense if it was "...jeez..." and not "...that's..."? That's what I always thought "mou" ment anyways. (Plus, it's better in this context)

I went with "that's" because the setting made me suspect that it might be part of an incomplete sentence (e.g. もうやだ, もういい) rather than a standalone utterance, but yeah, there's really no telling without asking the author directly.