Strabismus also known as Lazy eye is when one or both eyes to cross inward, outward, downward or upward. It can cause blurred vision and lack of depth perception.

This would be visible as a cub and would proceed even as an adult. And when a cub with the mutation is born it would have this message for example, "Your lion has gave birth to a little cub, you noticed their eyes are a little wonky...Maybe they would like a muffin?" (Yes I did a Derpy Hooves reference)

Because lions live in the wild it would not be treatable like how humans can be treated and since the lion would have blurred vision and a lack of depth perception they would be unable to hunt, patrol or become king but females would still be able to breed and the mutation would be non-lethal and they would live the same lifespan as a normal lion.