No, not necessarily. If you’d like to use a gallon sized zip-top bag as a makeshift piping bag that’s perfectly fine. Snip off a bottom corner of the bag after you’ve added the mashed potatoes and you’re ready to pipe. A medium to large hole will be best for piping potatoes. That said, to make pretty textures and shapes without a piping tip, a fork can be used to make swirly designs, or you can get creative and see what other kitchen tools you have at your disposal to make other attractive patterns.