It's really a good idea to get off the subject, and Mitsuri is actually pleased to hear Tamaki take the initiative on that. It means that he's not down about this to a debilitating degree.

So she perks up right away at his suggestion.

"That sounds wonderful! I'd love to hear you and your friends singing!" In her mind, she's picturing like... a vinyl or something. So she'll be shocked at how he plays them for her. "But I still want a performance from you too!" She says this last part with a pout - like she's trying to play up a childish tantrum.