Hair curl sponges are designed for textured hair, typically found on people of African descent. This is because the sponge's shape and texture are specifically meant to help create the defined curls and twists common in afro-textured hair.
However, this doesn't mean a hair curl sponge won't work on anyone with different hair types. If you have fine, straight hair, you might not achieve the same results as someone with tightly coiled hair. It's possible the sponge might create a slight wave or texture, but it's unlikely to produce defined curls.
Ultimately, the effectiveness of any hair tool depends on individual hair texture, density, and length. It's best to experiment and see what works for you, regardless of your ethnicity.