The fox mask Kokoro has on represents determination or seriousness according to Touhou wiki. So even if she can, she's determined not to.
Or maybe I'm just thinking too much and it's just her work mask.
Maybe she's determined to smile but that's the best she can manage
Even though it's on the menu!What's with all the yelling?This bitch refuses to smile for me!It's free, ain't it!?Smile, goddammit!!Come on! Gimme the smile!!KokoroCome on!!Kokoro