What could be better than using a surfboard as a sign to let customers know that this is a shop where you can rent a surfboard or get your "dings" repaired.
I guess this is similar to the practice of medieval shopkeepers in Europe who would hang metal signs outside their shops with shapes to show the type of shop it is because customers in those days could not read signs. Costa Rica has a literacy rate that matches the USA or Europe, so obviously this sign is intended as touristy kitsch.
There are other stores in Tamarindo that have signs made from surfboards. Tamarindo is a surfing town, after all. Although there are no places in Tamarindo to repair a car, and no gas stations in town (or the next closest town), there are plenty of places to rent or repair a surfboard.