Sakura would never put on an act - that just isn’t like her. Innocence is part of her personality, what makes her who she is. Aside from that…she looks more surprise, scared even in any case. I’d be scared too if Syaoran’s angry face was so close to mine. (Or rather, I’d be blushing like crazy...) And if I remember correctly, she probably didn’t do anything wrong maybe only sharing a smile with Yukito and because of it, Syaoran got pissed giving her that " back off " expression. This is likely the situation, but I’d need to watch the episode again.