I feel like the criticism by the director here is incredibly fair. It's very apparent if you've bene to a script reading before—the same parts in the script can be interpreted very differently by different actors. Even the same characters can feel very different, too. Some actors made their schtick their whole brand, even, to the point of getting typecasted.
Character fidelity when adapting something from a different medium is one thing, but that depends on the playwright and the director. Character interpretation is something that the actors add as a creative, and it's something that could not be conveyed through non-acted media.
Kozue did not give 'Kozue-ness' because she was so used to entirely resetting her personality expressions by borrowing from other people. She did not have Kozue-gestures, Kozue-looks, Kozue-tics, or anything that adds to the character beyond what was written and explicitly directed. She did not add how she feels about the character, how she thinks the character would behave, none of those. Like Mio analyzed, Kozue simply tuned things to the patterns of other people.
In this regard, there is no Yusa Kozue in the role, and that's the problem.