A mood drinker is a person who likes a lot of different types or taste profiles of whiskeys but when they drink they are often in the mood for a specific taste profile; and what their palate wants may change from day to day. For example, sometimes, a bourbon drinker may want something sweeter than the norm. Other times they may want a spice-driven straight unfinished bourbon. They may be in the mood for a complex finished rye or a traditional bourbon-tasting experience. It's when you like variety and are "in the mood" for a particular taste that day.
It does not mean how you feel emotionally (sad, happy, melancholy, etc.) determines what or how much you drink.