I'd say that lambs are more associated with docility than with peace, and it's a very fair association to anyone who has ever had any contact with sheep. Animal symbolism seems to have much more to do with animal traits, animal behavior or supposed animal behavior (e.g. strength with bulls, courage with lions and altruism with the pelican) than anything else and I see no reason to break that pattern. Furthermore, if it were the case that the reverence of some animals were due to degree is which they are useful, compared to the degree to which they are removed from preferred human ecological niches, then the most revered animals on the earth should probably have been fish.
