Wait, what's happening in the last panel? Who said Yuu-chan? Was Yuri just Yuu-chan disguised?
The idea is that Tomoko can affect Yuri's facial expression with her speech. The exaggeration of it is that Tomoko's speech can change Yuri's face, thus the punchline of Tomoko saying "Yuu-chan" to change Yuri's face into Yuu's.