In brief: Imperfectionism is a backlash against society's ridiculously high standards which force people to strive for flawlessness in every field. It is, however, not in the human nature to achieve perfection, which would in fact go against the philosophical definition of human being itself.   Mistakes are a fundamental component of our lives, and art was born for the need to represent our very reality, whether we are talking about the exterior, physical reality or the interior, psychological one. Therefore, art has become a part of ourselves and should thus mirror our true selves. Flaws do not ruin a piece of art; instead, they add to it a deeper introspection of the artist's persona.

The common understanding of perfectionism brought us to aspire to the same models, mostly rendering our art indistinguishable from one another, losing the freedom of creativity. Beauty cannot only be achieved by following strict rules; in visual arts, it shall be our eyes to decide what is harmonious and what is not. 

Summarizing what has been delineated in the above, imperfect art is everything that radiates beauty through mistakes.