What does this phrase indicate?
Some possibilities include
a) It doesn't indicate anything, milkmaids were no prettier than other girls
b) It is ironic, milkmaids were in fact notoriously plain
c) It was coined by someone who had a thing for milkmaids, or a crush onn a milkmaid, and was entirely subjective
d) Milkmaids wre indeed noticeably better looking than other girls
Assumming for the sake of argument we accept d)as the explanation, why should this be?
Again there are many possibilities to investigate, but some include
a) diary farms were established in areas known for their good looking women
b) dairy farmers only hired pretty girls as milkmaids, it was a clear case of discrimination against plain applicants
c) only pretty girls applied to become milkmaids
d) girls becoming milkmaids had much the same distribution of prettiness and plainness as the general population, but being a milkmaid made them better looking
e) girls becoming milkmaids had much the same distribution of prettiness and plainness as the general population, but kept their looks while those of the girls who were not milkmaids declined
f) some complicated mixture of the all of the above, plus perhaps further factors as well
In fact, e), though a bit of a strange one, is close to the truth. Milkmaids caught cowpox, a fairly mild disease, which protected them from the ravages of smallpox, a related, but much more virulent disease. From this Edward Jenner, though not the first to experiment with the concept, developed and popularised vaccination.