Two things, (since this involves me figured I'd jump in.)
First, Telling a person to calm down is like gas on a fire. "Relax" is probably the only worse option for some reason.
Second: Women (especially those blessed with RBF, like myself,) hear "Smile more" (and all the many variations) a lot, and I'm sure you didn't mean anything by it, (like you said, a joke.) Sometimes it comes off as rude (and/or creepy depending on the context.) I, seriously, just don't like how my face looks with a smile, so I don't and that's totally cool. If you like smiling, you do you.
I hope this came off in an explanation way and not an aggressive way.
Now, back to our regularly scheduled lack of hits.