I have had difficulty smiling for decades, especially when I am dealing with some internal concentration, such as trying to learn a new piece of music, or solve a mental problem. I see photos of myself and understand why people tell me I seem grumpy or antisocial.
But antipsychotics definitely made smiling more difficult and counter-intuitive for me.
When I have been off drugs, I range from grinning like a Cheshire cat to numb faced (most of the time) to being tearful beyond my emotions.

I think there is some merit in trying to appear more cheerful and upbeat (where possible) not so much because it makes us feel brighter, but because people around us get the message that we are nicer and easier to get on with. That gives us deeply needed positive feedback, especially when we are struggling with depression and the way people shun us for being mentally ill.

Obviously this is not very practical when we are in severe depression or psychosis, but I believe it can be helpful when we are in a better state.