They are not vegetarians according to a few sites
Muskrats eat a wide variety of vegetable and animal foods, including: cattails, sedges, rushes, water lilies, pondweeds, wild rice, pickerelweed, clover, willow, acorns, maple samaras, arrowhead, sweet flag, switchgrass, mussels, crayfish, frogs, snails, and fish.
http://www.fcps.edu/islandcreekes/ecology/muskrat.htm
Food and Feeding Habits
- Muskrats eat a wide variety of plants, including cattails, sedges, bulrush, arrowhead, water lilies, pondweed, and ferns. They also eat alfalfa, clover, corn, and other crops if muskrats find them in their territories.
- Although muskrats will eat shellfish, snails, fish, frogs, and salamanders, such animal foods are a small part of their diet, and are generally consumed when plant foods are scarce.
- Muskrats normally feed within 150 feet of their main dwellings; however, they will travel much farther in search of food.
- When muskrats become too numerous, an “eat-out” can occur where nearly all the available food is eaten. The eat-out area becomes virtually uninhabitable for muskrats, and only a few animals may be found where dozens or more once were.