While we have terms like "flock" for sheep, "herd" for cattle, and "gaggle" for geese, there's no established word for a group of mutton.
This is likely because mutton refers to the meat of mature sheep, rather than a living group of animals.
You could use a general term like "supply" or "stock" of mutton, or be creative and use a descriptive phrase like "a bounty of mutton" or "a pile of mutton. "