A want of animation or brilliance WordNet 3.0 & Open English WordNet

In art criticism of the 1960s and 1970s, flatness described the smoothness and absence of curvature or surface detail of a two-dimensional work of art. Critic Clement Greenberg believed that flatness, or two-dimensional, was an essential and desirable quality in painting, a criterion which implies rejection of painterliness and impasto. Wikipedia

Mattness, the quality of a painted surface which scatters or absorbs the light falling on it, so as to be substantially free from gloss or sheen. Wiktionary

Mattness. Wiktionary (translation)