Given the biochemical nature of most moods, it could be conceivable to develop a substance that, when ingested, makes people feel bored out of their skulls.
Yes, our experience of boredom is unpleasant. But the same can be said of many sensations produced by drugs: dizziness, hallucinations, impaired motor ability, etc.
Much of the appeal of drugs is that they allow you to take control over your mental state. Power is preferred to powerlessness, even when it's only power to make things worse. (Indeed, Freud postulated that this is the source of masochism. A person who sees pain as inevitable will seek it out in order to have some degree of control over the matter.) Boredom would appeal to this aspect of human nature very well, because boredom is a very powerful influence.