Katie was pleased with the first two champions, because Viktor is an amazing Quidditch player and Fleur is hot. (But she would have gone a bit red when Fleur's name was called, and avoided looking at her.) She was disappointed when Cedric's name was called because she'd been rooting for Angelina, but she would've clapped anyway.
When Harry Potter's name came out of the Goblet, Katie was just as confused as everyone else. But by the time she got back to the common room, she was well and truly feeling the party mood that was going on, and would have joined in the celebrations enthusiastically. After all, Harry is her teammate, and she'd support him wholeheartedly.