The theme of this film is tradition, which can be a good thing when it preserves continuity across generational lines, but harsh and restrictive when it allows no room for growth. Ironically, the skill of the "grandpa" is in surprising his audience with masks that are ever changing, yet change is the one thing he finds hardest to do. Tradition requires that his skills can only be passed down to a male heir, and hence he would sooner allow his skills to be lost than to break with that tradition. The message he ultimately has to learn is that tradition can sometimes be wrong, and that even he can be surprised by the unexpected mask.
This is a Chinese film, in which we are given the Chinese perspective, but the message is universal. On another level, consider the Christian perspective. Metaphorically speaking, what if Jesus came back wearing the mask of a little girl? Would that representation be rejected on the grounds that it wasn't what they were expecting? Would they reject the mask, and thus miss the message? Or consider the Aztecs of Mexico, who fell victim to the Conquistadors, because Cortez resembled what they thought was the return of their god Quetzalcoatl? Beliefs about traditions can not only be wrong, but potentially enslaving. When we become so blinded by tradition that we can see no room for change, change may have no room for us!
This is a marvelous film, which begs to be compared with "Whale Rider" (2002), having a similar theme but presented from the perspective of a New Zealand Maori tribe. They, too, had a tradition in which the mask of the leader could only be worn by a male, and when a male could not be found, would sooner the tradition die than change. The point of these stories, of course, is not the girl, but the change. There is more to value than gender. When tradition can only accept the one, it might be surprised by the other.