Why lightmap so dirty on Mobile?

This is a combination of low resolution lightmaps (which you fixed in later screenshots) and the mobile specific texture compression that is used called PVR texture compression. It has a much lower memory usage than the standard DXT1 but lower quality as well in some cases. There’s not much we can do here, we have to use the texture formats supported by the hardware. Most games cover this up a bit by using diffuse textures.