Excusez-moi?? Why should she 'attempt to change that'? I genuinely don't understand.
I'm also an immigrant from a very blunt, straight talking, no beating around the bush, no small talk culture. You'd probably be asked to leave the shop (or at least get a serious wtf stare) if you started gurning for no reason or engage the cashier in some random chat.
I suspect I'm ND, but not diagnosed.
I hate gurning like an idiot. I smile if I have something to smile about. Not 'just because'. I always say hello, please, thank you and bye when I'm in the UK. That's 100% enough when interacting in the supermarket/shops in my book. I do not and will not engage in pointless faux-friendly, time wasting 'chat' about nothing. They're not my friends and I couldn't give a monkey's what they think about today's weather or price of the tomatoes.
I obviously do not abuse staff, don't roll my eyes, keep a neutral facial expression, don't call them names. Unless they do it first, then it's fair game.
If that makes me rude, so be it. Being an immigrant doesn't make me a performing monkey who must ape every local behavior trait/social norm.