Heat a large skillet over medium-high heat. Cook and stir beef, garlic, garlic powder, salt, and pepper in the hot skillet until meat is browned and crumbly, 5 to 7 minutes. Drain and discard grease. Stir pasta sauce, Italian seasoning, basil, and oregano into ground beef mixture.
Ladle a generous layer of meat sauce into the bottom of a slow cooker; add a layer of ravioli.
Ladle another layer of meat sauce over ravioli layer; alternate with remaining ravioli and meat sauce until all ingredients are used.
Cook on Low for 3 to 5 hours. Sprinkle ravioli mixture with mozzarella cheese and continue cooking until cheese is melted, 45 minutes to 1 hour more.