Humans drink human milk, and the milk of other animals. Animals drink the milk of their own species on a regular basis. I will never understand why people get hung up on this in Zootopia as if drinking the milk of other creatures OR the same species is something humans and animals don't ever do in real life. It's like y'all just want it to be weird.
Humans don't usually sell their own milk to others or turn it into processed foods; it's reserved for nursing their own babies. There are no lower animals in Zootopia, they are all anthropomorphic and behave like humans. So them having ice cream is akin to human women selling their milk for the masses to consume. It's a little strange.
It's strange in our culture. Maybe in theirs it isn't.
Do people make breast milk ice cream? Probably. Whether or not it could become normal to be sold to people depends on how delicious it is.
Given that it's not normal, I imagine it's not delicious.
Human milk has a higher sugar content than cow milk so it probably tastes pretty good. I think the lack of supply is due to the taboo of it and the lack of lactating women who want to get milked all day