Shirley's lack of personality has been foreshadowed quite a lot, not just by her lack of soot, conversation, and by her relationship with Ram, but also in particular by the chapter covers of chapters 22 and 24, which show the Living Dolls and the Shadows in sitting position. These covers have some interesting details.
The Dolls all have the same posture. The boys hold their hands in exactly the same way and their legs have the exact same position, almost closed with the left foot going forward and the right foot going to the side.
The girls also hold their hands in the same way and their sitting position and leg are the same.
This doesn't happen with the Shadows. John has his arms crossed and legs open, Patrick has one hand on his chest and one leg slightly raised. Louise sits with one leg crossed and her hands on her knee. Kate has her hands on top of her legs and she's sitting with her legs sideways. Shirley, however, has the exact same position as Ram.
These details really show the difference in personalities the Shadows have and how (supposedly) the Living Dolls have no personality of their own.
The only exception is Shirley, who just like the Living Dolls, has no personality.
As a side note, the Shadows being influenced is a bit of a word play that is lost in translation. The word influence in Japanese is written with the kanji 影, which is the kanji for "shadow".