I agree with CyclopsSummers. I do try to smile at the neighbors, service people, people in the stores when we catch one another's eyes, or when they help me out--I am too short to reach a lot of stuff on the top shelves, so I often have to ask a taller person for help, and I will smile at children when I am out. I don't over do it though, just a quick smile, and maybe a nod, to acknowledge their existence and to appear more normal and non threatening.
I will never appear totally normal, as I don't wear make up, and am not a slave to fashion. Also, I am overweight, have bad knees, and have always had an unsteady gait, due to feet that were turned out some since I was born. Corrective shoes in childhood only partially fixed that problem. They were expensive, uncomfortable, and weren't doing the job too good, so my parents eventually let me get more comfortable foot gear in normal styles. I also have some scoliosis of the spine, that adds to my unsteady gait. My older brother had that worse, and had to wear a back brace for a couple of years as a teenager to straighten it. After they found out I had it too, nothing was done for me. I don't know why. Perhaps my case was not considered serious enough to need a brace.
I realize many of us on the spectrum don't see the need of smiling, and often don't have much to smile about, but it doesn't cost anything, and can help make the NTs more comfortable around us. That can make things a little easier for us. In my case, I am also trying to combat the negative effect of my eye brows. I tend to look angry no matter why I might be frowning. If I am not frowning, the original brow line was a clown curve--not a help. They also tend toward spikey and bushy. I have worked to adjust my eye brows over decades, to appear less clownish, less "angry", and less bushy. I have made some improvement in it, but am hampered by the fact that the follicles do grow back. I don't want to simply remove them and use an eye brow pencil, as I am not into wearing any kind of make up. Besides I don't like the look of fake, drawn eye brows.
You should just let it ride when people are smiling, and you should consider the quick smile, with perhaps a nod, yourself, to help you get along a little easier with NTs.
_________________
If a man does not keep pace with his companions, perhaps it is because he hears a different drummer.
Let him step to the music which he hears, however measured, or far away.--Henry David Thoreau