Shape key rigs are typically used for facial animations or for creating morph targets. The colliders and default cloth settings need major adjustments, but it turns the cape mesh in to a flowing version of itself.I would love to see the full body topology without clothes. For rigging capes I was using Blender (free, open-source), but lately I actually have been having success with just importing a cape object (*.obj, *.3ds, *.fbx, etc.) into Unity, attaching it as a child of the object 'wearing' it (the capeless model I rigged in Mixamo), and then adding a cloth component to the cape object. One problem with Mixamo is that it only does humanoid models, with two legs and two arms, although you can try non-humanoid models but they get deformed. It deserves credit, they even offer a massive collection of free animations and models, all nicely organized and textured, and compatible with Unity. They may have removed some features, like the face-animation scripter, the mesh-morpher, the online storage space(they still allow 1 at a time stored online) and my favorite tool, the Decimator which was a great tool for reducing mesh file size (polygon count), and a couple of other minor things, other than that it's actually the fastest and easiest way to rig a humanoid model for use with Unity, and it's free. Mixamo has been updated several times, improving Unity-compatibility (they have a Unity-specific FBX format).

Auto rig pro mixamo