AB has curly hair, really curly hair, and strangers comment on it all the time. My stance on this is to tell her “it’s not really polite for people to randomly comment on your appearance, but it’s not impolite enough for you to be impolite back — just say thanks and move on.” Is that the right stance?
Anyway, though, today someone at the farmer’s market said “I would die to have hair like yours” and AB said, in a non-combative, sunny way, “How would that help you if you were dead?” and I was super proud.
Quomodocumque 
Regarding your question: Why would it not be polite for a someone to compliment a stranger on their appearance? I once got the compliment that someone liked the scarf I was wearing, and it made me feel good. Generally I think that’s what compliments do to a person, they make them feel good…