My dog has a soft, jelly-like lump on her cheek. Is it a sebaceous cyst?

Hard to tell based on the photo unfortunately. Masses that are underneath the skin, like this one, have to be felt in person. If it is soft, then it is likely a lipoma (benign fatty tumor), but it is best to have your vet feel it and potentially have it tested to make sure. It definitely isn't a sebaceous cyst, b/c it is not a dermal (skin) mass but instead is underneath the skin.
Best wishes with your pet Missy moving forward. Take care!