of course it's sexist. He most certainly wouldn't tell a passing grumpy-looking bloke to smile. it comes from a place of feeling entitled to benefit from women: that the point of a woman's existence is to be pretty and pleasant and make men feel better. very much sexist. she was right to complain and right to tweet about it.
linking it to suicide awareness is a separate issue and muddies the waters unnecessarily.
but every single tweeter telling her to get a grip and that it was reasonable to tell her to smile is themselves also a sexist arse. There is a lot more misogyny in the world than we would like to think.