1) The vertex count on your skeleton's meshes are actually a bit high, and it also looks like a lot of bones are driving the shape of the shadow. This can worsen render/mesh update performance.
Depending on how big this character is on your target device, the benefits may not even be visible. This also means the geometric clipper code needs to go through more triangles to perform its function.
2) It looks like only two of the animations area actually using the clipping. (sale, and entra trampilla). The rest are not. But as long as the clipping attachment is active, the renderer will perform the clipping calculations.
It's better to disable the clipping attachment whenever it's not used. Only enable it when it's used.
3) On both sale and entra trampilla animations, the masking you're trying to achieve is actually really simple. You'll definitely benefit from doing this the shader-stencil way on the Unity side, rather than using Spine's clipping mask.
We were looking into supporting SpriteMask but it seems to be a purely shader thing, especially since Unity doesn't expose some of the render API that allows us to just change stencil test values without needing a new shader similar to how SpriteRenderers achieve this.
If you are just using the Spine/Skeleton shader, you can use this as a replacement to become compatible with UnityEngine SpriteMask system: https://gist.githubusercontent.com/pharan/eb758e46630ad7ae2234227480922c21/raw/63aa60a7f1bb8ef3ce66161786b9fd5de3877aab/Spine-SkeletonSpriteMaskable.shader
Depending on the values, you may also need to change the stencil values on the material/shader.
If you're using a custom shader, you'll have to add the stencil test into it yourself.
See the discussion on it here: The discussion so far is here: [unity] Unity 2017 SpriteMask support · #941