Hi,
If i understand the following correctly
unreal.DNAAssetImportUI — Unreal Python 5.4 (Experimental) documentation (unrealengine.com)
unreal.DNAAsset — Unreal Python 5.4 (Experimental) documentation (unrealengine.com)
You should leave this to empty if importing the new metahuman (new sk mesh will be generated then) or chose the existing one (most likely the FaceMesh) when updating the one that already exists in your project.