Who decides whether art is good or not?

Short answer
Glenn D. Lowry.

Long answer
Quality is a moniker for what the people you respect appreciate. Even if you do not agree with them, there are those who, you perceive as authorities in the field on account of their expertise, their position or the respect they enjoy among their peers. Subsequently, what is considered geniality changes as those people are substituted. The Pre-Rafaelites were considered the best things that had happened in the art world, but well into the twentieth century nobody defended them any longer and their work was automatically devalued as passé. The paintings themselves did not change over time. Quality is a relative term in a similar way as Time (which feels absolute in the present moment).

Question # 53 from “What is art” (must buy) by Ernst Billgren