Can I teach her to mellow out for all general public situations??
I guess that depends on what you'd expect her to do in a 'mellow' state.
When people get excited, our heart rate goes up, adrenaline may kick in, a whole bunch of changes take place. Something similar likely occurs in animals, and I highly doubt it's possible to teach a dog to calm itself down. Best case is self-control and restraint in a (still) excited state. And for this, I do like the down-stay.
If my girl gets overly excited when meeting strangers, I put her in a down 5-6 metres away from the people/dogs we just met, and I go over there to talk with the owners and play with the dogs. She stays put and is generally calm. However once I release her, then it's game on (unless I release into heel). This works for me, but it's more along the lines of management than a true mellow state.
If I'm not planning to walk over to the distraction, then I just put her in either sit or down and demand eye contact until the distraction goes away.
The best solution would be to not let the dog get into an overly-excited state to begin with. Teach her to ignore the world and focus on you.