You are reading a scientific article about cheetahs that have stripes instead of spots. the article refers to this as a genetic mutation. though you see they have no negative impact on the cheetahs fitness, the stripes are nevertheless considered a genetic mutation because mutations:

Mutations may also impart beneficial genetic traits to living beings.

Explanation:

Mutations are random changes in the genome of living beings. Mutations are directionless, that is, mutations can be beneficial, harmful or neutral for individuals. The presence of stripes or spots on the skin coat in cheetahs is a genetic trait and is regulated by genes. A population of cheetahs with skin spots may develop stripes as a result of genetic mutation.

The new genetic trait imparted to living organisms by mutations may increase their reproductive and survival fitness (for example antibiotic resistance gene in bacterial population), may be harmful and cause some disease (example: sickle cell anemia) or may be neutral.