No, they are not all like that at all. There are different lines of breeding. Most photos are of the lines of breeding for looks and size. The TM's in Tibet are quite different and not often photographed. Most TM's you see in China look closer to the older Tibetan style working guard dog rather than that dog in the photo.
I'm not a TM expert but I do live in China and TM's are common here for a large breed. They are the most common mastiff breed you will see here.
Like anywhere, there are quality breeders and byb, puppy mills etc. there are some breeders in Tibet that will not sell to non-Tibetans including Han Chinese.