Popularity is determined through achieving certain standards in your café. This is the measure that ensures a healthy customer flow in your cafe. The higher the popularity, the more customers will enter your cafe. A cafe can reach a maximum of 100 popularity, and will be decreased by different shortcomings in your cafe. If your cafe remarkably exceeds the standards set by the game, your cafe will have a static 100 popularity. Richly decorated cafes are known to have static 100 popularity. Parties also give your cafe a static 100 popularity while the party is ongoing.
For more information, head on over to the Cafeland Wiki.