Addon for Blender 2.93 and 3.x for transferring shape-keys from source object (body) to target object (clothes).
Location: Right sidebar -> Tools -> Shape Transfer panel.
How to use:
Video tutorial
Enter Source object name. Source object should have shape keys (eg. body with fat, medium, slim shape keys).
Enter target object name - that will receive shape keys (eg. clothes that will receive fat, slim, medium shape keys).
Press shape transfer - you clothes will receive shape-keys from body object. Changing shape key value slider on Source object will now change corresponding shape key value on your targe object. You can delete shape keys with - "shape keys clean" button.
While there are no restrictions with regard to the topology of modified mesh, the source object’s mesh (usually character body) has to follow few requirments:
It must not contain edges with more than two faces.
It must not contain concave faces (no longer required since 2.5 update)
It must not contain overlapping vertices (doubles).
It must not contain faces with collinear edges.