Dem. Sen. Claire McCaskill says HRC should smile a lot during debate. When I said that, I was called a sexist. https://t.co/U9T5GnZJe1— Brit Hume (@brithume) September 16, 2016
It depends on how you say it. And in this case, it depends on whether we think you're actually trying to help or just criticizing her — and whether criticizing you seems like a way to stir up energy in support of the woman.
But in general, it seems that people expect more sociability and openness from a woman and are more likely to feel uneasy it she keeps a stern or grim face. The relaxed female face gets the jocose description "resting bitch face." So serious women can have trouble being serious as people express what might only be a simple desire to overcome their own uneasiness but might also be a more virulent urge to exclude women from serious enterprises. So it's naive to become another one of those people who tell women to smile.
But smiling is still a great facial expression that most people ought to want to know how to use to their advantage, and some advice to smile is good and not always sexist. Other people are absurdly responsive to smiling.
Here's my experience. I do a lot of walking around town, an average of 4 miles a day (according to an app in my iPhone), and I'm nearly always listening to audiobooks, and I tend to listen to serious things. There was the summer I listened to "The Rise and Fall of the Third Reich" and all the many months it took to get through the 4 volumes of Robert A. Caro's biography of LBJ.
I don't look at my face when I'm walking, but I assume I look fairly grim when listening to that sort of thing. But sometimes I put iTunes with a shuffled playlist of songs I'm pretty sure I'm going to like to hear if they pop up — The Monkees "I Wanna Be Free," Herman's Hermits "Dandy," Elvis Costello "Watching the Detectives," Cher "All I Really Wanna Do," etc. etc. Again, I'm not looking at my face. But I know I must be smiling, because suddenly, on my usual walk, everyone I pass looks at me and smiles, like they're seeing an old friend.