Here's why:
* Mutants are a visual change, not a separate species: A mutant Neopet is just a regular Neopet with a different color and/or pattern. The clothes available are designed for all Neopets, regardless of their mutation.
* Clothing is based on species, not mutation: Neopets clothing is categorized by species (e.g., "Kacheek Clothes," "Ixi Clothes"). You can dress your mutant Neopet with clothes designed for their original species.
So, if you have a mutant Kacheek, you can still dress them in clothes designed for Kacheeks!