Yes, it is safe to eat marshmallows cooked over propane, as there are no harmful substances that could affect your marshmallows. However, other factors can affect the quality of your marshmallows, such as the type of propane you use and how hot your stove gets. If you're using a standard, non-stick pan, you'll want to ensure that your stove is set to low or medium heat. If your stove gets too hot, it can cause the marshmallows to burn. It's also important to keep an eye on the size of your marshmallows. They might not cook evenly and become hard and crunchy if they're too small.