If you have never played at a coffee shop before, let me tell you a little about it. I am doing this so that there is no shock.
First of all, the customers are there, mostly, to socialize in one way or another with their friends. They bring people along with them. The music that is performed adds to the atmosphere of the environment. Simply put, it is an element that enhances the background upon which the people socialize. This means that some of the people ARE NOT there simply to hear the music ALONE.
This creates a situation where people may/may not clap at the end of each song, or set. This DOES NOT mean that they do not enjoy the music, or that they do not appreciate the effect the artist's music creates. It simply means that the conversation they are having is the most important thing to them.
Most people, periodically, will clap upon breaks in the conversation, though.
I hope this will clear up any misunderstanding about the way the crowd will react to performances. Simply because they do not show it, does not mean they do not enjoy it. Nearly ALL of them do love having musicians play for them!