Mild:
* A bit rough: Implies the food is not great, but not disastrous.
* Home-style: A more polite way to say the food isn't fancy.
* Not quite restaurant quality: Points out the food is not professional standard.
Moderate:
* A bit of a disaster: Suggests the food is not good, but not inedible.
* Burned: A common term for food that has been overcooked.
* Undercooked: The opposite of burned, food that wasn't cooked enough.
* Over-seasoned: Too much salt or spices used.
Harsh:
* A culinary crime: A very strong way of saying the food is bad.
* Uneatable: Means the food is so bad that it cannot be eaten.
* Disgusting: A very strong word for food that is revolting.
* Dog food: A derogatory term for food that is very bad.
The best slang word to use depends on the specific situation and the level of negativity you want to express.