I’ve been using the Mixamo Fuse Basic program creates an avatar and automatically rig. In the mirror view the presentation of the avatar is not smooth. You can see structures at the avatar. The presentation of the avatar spine is not ok too (dark stain).
That more than likely is the shadows that look odd on avatars backs. If you turn off shadows you will get it looking better I think.
I have turned off the shadow. The structures at the avatar are disappearing, but the dark stain at the spine of the avatar can still be seen. Thank you Coal.