I agree with Hitosura said in the last post
I'd say the ratio would be something like:
25% pure shitheads: nothing to say about these, they're like this since even before birth
25% pure niceheads: while the name might sound compelling and desirable for a pet, these are the kind of yukkuri that are too nice for their own good. Just to make an example, these would be the kind of pet that would willingly open the window/door to a stray family because "they are a pitiful family", most likely ending up dead
In the wild these usually die soon while helping their shithead sisters with some trivial matter that the sisters can't do
The remaining 50% is, as Hitosura said, those that are born neutral, but behave in a certain way because they don't know any better
It could be from being spoiled by their parents, or even just doing what the parents do (because they were shitheads), but they are those that can change through some heavy training
Yukkuri as food, I talked about them earlier, are naturally cheap as you don't have to do anything in particular to get them. You don't have to train anything and even shitheads are fine, so no particular quality control is needed
About taste, if I recall correctly, in the comic with the kid visiting the factory, the food was made with freshly born yukkuri
I personally believe yukkuri get "taste" after growing a bit, so newborn babies are bland, while koyukkuri and adults have more taste in their fillings, but of course this is headcanon so take it as you want