Please explain “why” you are doing this. I assume you are trying to modify/customize the metahuman cloth at runtime for whatever reason.
Caching the “material” would require caching all the textures used by the G-Buffer to be done correctly. However, the Unreal RenderTarget does not do that but instead renders an “unshaded” quad only outputting the emissive color to the RT. Try wiring your output color to your emissive channel. In case of Metahuman you need to go down to the parent shader.
I have tried outputting the cloth base color of the Metahuman shoes to an RT and it worked out of the box.