I don’t believe it’s important for cartoon cloud 
Although I wonder how to achieve that. Vertex color? (But then how do I darken part of the mesh using vertex color, on material side I mean)
I don’t think it’s possible to replicate original images in Blender. I think I can live with clouds like what I came up with
There is nothing special about the technique. Model base shape, subdiv to archive high density of the mesh, displace it, add material. Material is Toon shader mixed with Velvet shader. Turn on AO. That’s all
For a quick and dirty SSS-like effect that gives you a lot of artistic control, I’d use three materials in a node shader blended with add mode:
-your main diffuse/specular material, it’s fine if this has hard edges
-“shallow” front-face SSS (for light that scatters across the surface and back out the same side)- use a toon shader, and adjust the size and softness. You can adjust the color of the material to control what color of light gets scattered back out. This is particularly good for light passing around the edges of the clouds.
-“deep” back-face SSS (for light that passes straight through the object while scattering)- take the normal output of the main material, run it through a vector mapping node that inverts the vector, and plug that into the normals of another toon shaded material, this will render the light on the other side of the material. You can then bring the softness way up, and the brightness way down, and change the color to again change the color of the scattered light.
In addition, if you want thinner parts to let more light through, you can invert the normals of the object and bake AO to vertex colors (and then re-invert the normals so they’re facing back out again). The result is a functional and quickly-generated thickness map, that you can multiply into your back-face SSS color, so thinner areas will let more light through than thicker areas. Since it’s baked to vertex colors you can also do some manual painting to adjust it as you see fit.
The result is less realistic than proper SSS simulation, but it gives you as the artist a lot of control over the results.
Thanks for the advise! It’s just a way over my head 
I found there are only 2 mixing shaders - Mix and Add, and Mix doesn’t really work as anticipated out of the box 
Could you please make a node tree for your example, if you have time (including with thickness map) ?
Oh! Right, I forgot you were using Cycles, and I was talking in viewport/internal node language. For non-realistic rendering I personally prefer internal rendering, just because I understand the way it works a lot better so I have a much easier time getting the results I expect (and most of my work is intended for real-time rendering, and the internal renderer is much closer to that than Cycles).
Anyways, making clouds like that seems like a neat challenge so I’ll take a stab at it when I get home, and success or not I’ll share my results.
I appreciate that, thanks!
If it doesn’t quite work in Cycles, I can roll with BI. Cycles just much faster and doesn’t require much of the lighting setup (plus texture baking is a bit more robust in Cycles). It would be cool to have both setups for comparison.
Whelp, I forgot how unstable node materials are in Blender, and after the fifth or sixth crash I decided to call it quits. My results are not great, but I said I’d show them so here they are. These clouds are slightly-curved planes subdivided and mushed around, given lumpy normals, with a cloud texture blending in the edges and the SSS nodes. It should all render in real-time just fine, though the alpha is screwy because Blender does real-time alpha like it’s still 1995 (which is to say, random clipping and basically no sorting available.
The nodes are a mess, and annoying to work with if you want to make adjustments, I might start fresh and try again some other day though.
I’ll upload the file later in all it’s buggy glory, right now BlenderArtists isn’t letting me upload blend files. Not sure what that’s all about.