It wasn't a usual surprise to see everyone paying attention to Kousagi, specifically if Kousagi is another child of Usagi and Mamoru.

Chibiusa could never stand it if someone other than her was getting more attention. And Kousagi, was definitely enjoying the attention she was getting.

Chibiusa was trying so hard not to lose it, it's not surprising, she doesn't want anyone getting more attention than her.