Since villagers (at least those with which one can trade) must already be individually tracked, this doesn't seem undoable or particularly expensive (depending, of course, on how many skins and of what complexity need be stored).
On the other hand, I don't see enough utility to this that it would be high on my priority list…
[As an aside, I am firmly in the camp of those who reject the idea of purchasing skins (with an exception for those used by profit goaled servers).]
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WARNING: I have an extemely "grindy" playstyle; YMMV — if this doesn't seem fun to you, mine what you can from it & bin the rest.
It would add to creating ones own village, or story creation for maps, I just think maybe if you hit the bumper buttons when near an npc that a small screen should come up asking what to do changing skins could add to a lot of creativity. And it would give skins another purpose.