YES, I can tell what mood when someone walks in at my job office in the morning by their facial expression and posture.
Nonverbal Communication is very important when dealing with customers. When dealing with customers it's good to make eye contact and smile when required, that way the customer is attentive and is taking in the information with the aim of dealing with it. It's good to be mindful of your body language as each movement can show feelings to what you are saying with your words.
I use to work in retail. You have to walk around and see if customers needed help. One time I walked up to a lady to see is she needed any help. Soon as I open my mouth to see if she needed help, she gave me an ugly look and walked away. My nonverbal communication was not rude, I was smiling and tried to make eye contact. I just look at it as she was having a bad day. Her nonverbal communication threw me off.