Not necessarily, but sometimes a smile can be a little complicated.
Have you ever been in public, and someone unknown to you catches your eye? Maybe they look incredibly fabulous, or they carry themselves in such a manner you find yourself staring, perhaps with envy or even fascination.
Maybe you smile at the exciting stranger, and they will probably smile back in an innocent exchange of regard. It's all harmless, a "feel good" exercise for both parties, and that's fine.
However, please be aware that there are particular situations and cultures where an engagement like this, although fleeting, would be inappropriate. Be mindful of your environment and behavior accordingly and with respect, and you'll be fine.
If you find yourself in a situation where you're receiving uncomfortable smiles from strangers, remove yourself. The old saying about "going with your gut" has some clout, so acknowledge that and stay safe.
Or, you could be in a safe space, but you don't like some stranger smiling at you. It is your prerogative to smile or not to smile. There are no judgments, no etiquette missteps at play; it's simply your call.
But, for the most part, if a stranger flashes you a friendly smile on a day when you look amazing, smile back—it's a boost to you both. Accept the compliment, and return with a smile. Heck, it's like being kind; it's free; so go ahead and flash those pearly whites if you're so inclined.