If the skins they sell are higher quality than the ones available with mods, then people will buy them anyway.

If the mods are making those skins available for free then that’s probably a copyright infringement issue (assuming the assets are only made available once purchased, and that they’re not just available but disabled) that should be handled with a DMCA takedown.

If the mods are making their own content available for free but the mod content is lower quality than the paid skins, then the people who don’t purchase paid skins as a result will be far more limited. The bigger the quality divide, the lower the impact.

If they can’t compete with free cosmetics then that speaks to a lack of quality on their part.