That can be done rapidly using Tissue addon (which is included in standard Blender distributions, I think).
- Add a mesh.
- Eventually use a decimate modifier to reduce amount of vertices and add some randomness
- Use the "Dual mesh" button from the 'Tissue Tools' panel
- Add a wireframe modifier and a subsurface modifier
- 1 Add a mesh
- 2 Set wireframe display on so that you can preview the result
- 3 (optional) Add a decimate modifier and tune the ratio
- 4 Use the "dual mesh" from the "Tissue" addon options
- 1 Add a wireframe modifier, tune the thickness and if needed use the 'relative thickness' option.
- 2 Add a subdivision surface modifier to round the result.
You can lead the amount of decimated vertices (using 'decimate' modifier) with a vertex group.
So if you define a vertex group, then draw on it, that will allow to choose if Voronoi cells will be larger or thinner.
Here for instance, more decimation for the head and less for the bottom part or ears:
From left to right: vertex paint, decimated result, final mesh.
To do it on part of the model only (should work on quad meshes)
- Select the wanted faces (you'll need to keep this selection along the steps).
- Make a base for the tesselation (a square with a hole below that fit to quad faces)
- Tesselate using Tissue with a limited tesselation on selected faces
You now have the part prepared for Voronoi.
Now in the original, in edit mode, as the faces are still selected, delete this faces only (X then 'only faces').
Join the 2 meshes, and in edit mode merge vertices by distance.
Add a solidify modifier (as this mesh is flat and non manifold) and a subdivision surface modifier. You now have something like this: