My spectrum sons (the older one especially) can look angry when really, they're just neutral. Or even feeling moderately content. I'm used to t hem, so I don't see it, but friends tell me, "I saw your older son walking through the mall, wearing all his back heavy clothing and biker boots like he does, and I wanted to say hello but he looked so angry, I didn't want to go near him."
I pointed out to my friend that if he was feeling as angry as she thought he looked, he wouldn't have been anywhere near the mall, with all the bustle and people.
It DOES take effort to actually smile. They say it takes less effort to smile than frown, but so often we forget, that no expression is least energy of all.
As a NT, I would recommend you don't change who you are, but be aware maybe, that when you are greeting someone you may need to smile, a little. My Aspie husband used to tell little jokes, totally deadpan. But with absolutely no expression, people thought he was being serious, they thought he was really weird to say such a thing. So he learned to smile, just a tiny bit, when he made a joke. It works!
My older son's best friend (Aspie) really scared the teachers at school with his totally expressionless delivery. He was physically imposing, which made them even more nervous. One nasty teacher eventually used that fear of him to get him thrown out. And being Aspie, he was ill-equipped to fight back in any way (especially legally).
You may find family won't comment because they know you are can 'read' the much smaller expression changes, but it will be friends and strangers who won't understand as well.
Also be aware - some people will say to ANYONE who for that moment has an expressionless face, "Cheer up! It probably won't happen." Or something similar. it's a joke, a light-hearted way to change a topic or even begin a topic. A conversation starter, a chance to share something that might be bothering you, or a chance to say something light-hearted in return, such as "let's go catch a movie."