Food is a daily struggle for most people. The wealthy and powerful hoard the food whenever they can, but sell/trade surplus. The brave and strong are able to go out into the Citywilds to hunt and forage.
Farming is limited to lands that people can protect from treeplague with brute force (hacking the kudzu down every day) and with spells. That’s mostly within city walls, so we’re not talking thousands of acres. The feudal system crumbled overnight when the miles of sprawling manorial estates became wilderness.
Largely, people subsist by a combination of personal gardens, milking small dairy animals like goats (which eat just about anything), raising hardy meat animals like hogs (which also eat just about anything), foraging for new and exciting plants, and hunting new and exciting animals.
Yes, the mutations include flora and fauna. Insects are becoming more of a staple. There are lots of weird fungus and root species that are tasty and nutritious though some cause unwanted magical mutation. There seems to be a surfeit of dragons, though most are bird-sized. These are quite tasty, and most are reasonably easy to hunt (and none are intelligent species).