I made a mudokon Sim a long time ago, though I have yet to perfect a skin. Here are some screenshots for you!
(Phil in the editor)
(dancing like a dork)
Here are some tips to making it (and if you like, somewhere I can upload the skins I have, though I want to make improvements).
-in the editor, you can change the sim's facial features, but they can only be deformed so far. HOWEVER, if you save a face deformed as far as it can go and then open the editor again and clone it, you can get it to go even further. This is how I made a mudokon face, by deforming the eyes and making the nose and chin almost entirely nonexistent.
-For the hair I'm still having trouble, but I took the hair of the bald guy and colored all but a strip the same green, so that he had a strip of hair left on the back to simulate a ponytail. I'm looking for a meshed ponytail though, that would be better.
-for the skin, make sure to get all the views and ages... that was my problem. Now I can't let Phil, my sim mudokon, grow old because he'll turn flesh-colored. You can add whatever sorts of wrinkles and mottling you like in any paint program.
-For the loincloth, I just changed the color of his swimming trunks to brown.
I hope this helps!