Hello, thank you for the comment! The diamonds indicate 'stars', which correspond to the colour of the card as well. 'one-star' is yellow with one diamond, 'two-star' is red with two diamons, and 'three-star' is blue with three diamonds. There are 2 one-star monsters, 3 two-star monsters, and 4 three-star monster.
The diamonds are a way to easily identify what star rank the monster belongs to even if you print in black-and-white or have colour blindness.