I don't think there is much to do. If the doctor says her weight is okay, then it's fine. My DS was always low on the growth charts (still is) and eats like he's not going to see food for another 2 weeks. The smoking, while it really sucks, isn't illegal. My kid also hates strangers and always has. When he was little, he would cry around anyone he didn't see on a regular basis. Now, he will still stay away from them, not smile or talk, etc. (even though he doesn't stop talking on a regular basis, lol). It's just how he is. He would be the same way around random neighbors.
Of course if you see things like her riding around without a car seat, or anything like that that really impacts her safety (and again, I realize smoking can impact your health, but there is not a whole lot you can do about it), then by all means get involved and call CPS. But at the moment, I just don't think there is really much to be done over neighbor speculation/hearsay.