The Smunkin Patch in My Singing Monsters is a decoration that will help boost your income for certain monsters and will allow you to decorate your island. This decoration is normally used to make your island even better and is unlocked into your game only at certain levels.
Decorations are generally blanked out until you have reached at least 1 level prior to its unlocking. This decoration will also have a set range which will allow certain monster’s earnings to be boost. The boost will only affect the earning rate and not the max coins held by the monster in your game.
For players wondering if having the decoration liked by the monster in your island will make it easier to breed for that specific monster, then the answer is no. Unfortunately, there is no proven results that suggests that having certain decorations in your island will increase the breeding rate of any specific monster.