There is no way to know whether mashed potatoes are an inferior good unless we have access to information about the quantity demanded of mashed potatoes and how it correlates to consumer incomes. Inferior goods are goods that are more in demand when incomes are low and fall out of favor as incomes rise. This is the opposite of normal goods, for which demand rises as incomes rise.
We can make conjectures about mashed potatoes. We can argue that they are inferior because people with higher incomes will prefer baked potatoes. (This is especially true if you are talking about the instant mashed potatoes that you buy and reconstitute.) But we cannot know for sure whether mashed potatoes are an inferior good without more information.