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 issues in your cafe.
Static 100 popularity
If your cafe remarkably exceeds the standards set by the game, your cafe will have a static 100 popularity. This means, no matter how many issues you have in your cafe, the popularity will always stay at 100. Richly decorated cafes are known to have static 100 popularity.
Parties also give your cafe a static 100 popularity while the party is ongoing.
These issues decrease a cafe's popularity and are generally recommended to be fixed in order to keep the popularity score up. These can be found by clicking the popularity meter, or looking at customer's thought bubbles.
Why it appears
How to fix
-1 per stain
There is a patch of dirt on the floor. It can be a water spill, coffee stain or a patch of footprints
There can be a maximum of five dirt patches in your cafe, regardless of cafe size
The game considers not only floor and wall decorations, but the cost of everything in the decoration tab (this means, including chairs, tables, wallpapers, floor tiles, counters, etc.)
Quality > quantity. If your cafe uses expensive items for its basic necessities (chairs, tables, counters, stoves), the "increase your decoration quality" may not pop up even without a single floor/wall decoration