The more we become detached from thinking of meat as animals the harder the impact of realizing it again could be, but if it became normalized people would just stop being shocked quickly.
[0] human child not goat
That said, lamb is quite rare in Japan, it's eaten primarily up north in Hokkaido.
Lamb is meat from a young sheep - raised to be eaten young.
Mutton is meat from an older sheep, generally from sheep no longer good for wool production, too old to bear lambs, etc.
Mutton is a relative rarity outside the farm gate in shops and city butchers .. in an economic sense as soon as a sheep is big enough and well fed enough to be sold on to super market chains, why invest further time in that animal?
Unless, of course, wool production and| lamb production (ie. older ewes and some rams).