Sure, it's almost certainly moddable. But read the Mongol DLC description. The DLC adds thousands of portraits. Now go read developer diary #7. How many dozens of hours do you think it will take you to make that many portraits yourself?
It is few variations of noses, eyes, hair, etc. that gives potential amount of thousands combinations - not thousands of unique portraits.

Single modder could do the same set in say 20 hours of work time, although faces are difficult to make and I doubt modders would replicate Mongol faces DLC with similar quality. Clothes, hair, headgear and other portrait elements on the other hand are, and would be modded. However, since paradox decided to hide modding forums for casual players this time around, restricting it to small percent who bothered to register on the forums and then register game, I'm sure these paid DLCs would sell. It is even more apparent with 'Dynastic CoAs' DLC - it requires almost no skills or huge amounts of time to mod such a thing.