Art is a subset of the class of objects that are created by people and have an effect on the mind of other people. It may or may not have many other definitions and features, as people have argued over for millennia, but this is a true thing you can say about it. There are many mind-affecting things not created by people, but they are not art. The selection, curation, or framing of things not created by people might be art, but this is done by people. Furthermore, I believe everyone can agree that something that has no effect on the mind of any observer is not art.
So, no matter what other judgments you can make about art, you can always make judgments about it that you could apply to any other object in the class of things that are created by people and have an effect on the mind of other people.
The most relevant judgment is what effect it has on the mind of people. What are some general features of that effect, and is it a positive or negative effect?
One effect that it can have is to give people pleasure. Not all things that give people pleasure are art, and not all art gives people pleasure, but to the extent that art gives people pleasure, it can be judged by the standards of such pleasure giving objects. In many cases art, or a reproduction of it, gives a great deal of pleasure for a relatively low cost, so in that respect it is quite good.
Another effect of art is that it can teach people things. Not all things that teach people are art, and not all art teaches things, but to the extent that art teaches people things, it can be judged by the standards of things that teach. People can be taught things that are true or false, good or evil. It is legitimate for society to encourage the production of things that teach truth or good, and discourage the production of things that teach falsehood or evil. When dealing with art in this way, it is legitimate to focus on what is being taught and ignoring other aspects of it.
A third function of art, perhaps its main function, is to define and reinforce social boundaries between different groups of people. Not all things that define social group boundaries are art, and not all art serves to define a social group boundary, but to the extent that art is being used as a social group boundary, it can be judged in that manner. Because cohesive social groups are valuable to people, anything that helps create such cohesive social groups adds value to society.
However, there are better and worse ways to create and define social groups. The best way is for everyone to admit that their preferences are arbitrary, and their subculture is simply a collection of people united in enjoyment of an arbitrary thing that is neither better nor worse than other people's taste or fashion.
Things become more costly, in the sense of generating negative emotions and status anxiety, when people insist that one social group or subculture should be accorded higher status than another. In general, almost all arguments over what should be defined as art are actually arguments for one particular group of creators or consumers to be labeled as higher status than another group. The word 'art' denotes high status, so people spend a great deal of effort inventing impressive arguments for why the thing associated with their group is art, but the thing associated with somebody else's group is not art.
Things can become very bad when a subculture unites around a definition of art that is either deliberately ugly (meaning that is designed to not give pleasure to the typical person who views it), or teaches things that are false or evil. This can happen quite often because of countersignaling. In such a case, the broader society has a right to tell the subculture that they are simply wrong, and should be considered low status.
A very important confounding factor when discussing art is that people instinctively enjoy and appreciate a thing more when it is especially difficult to create. The difficulty of creation serves as a costly signal, and whenever a thing serves as a costly signal of quality, because it requires expensive ingredients or a great deal of training to produce, people will instinctively consider it high-status and wish to own it and affiliate their group with it.
However, this difficulty of production should always be counted as a cost rather than a benefit of art. It would be better for society if people got the benefits of art out of something that was much cheaper to produce, and the expense of production is very often nothing more than a costly and wasteful signalling game. People try to attract attention to their thing by paying for really high production values, but that simply makes everybody else's thing look relatively worse, without any net social benefit.
Art may have many other features, and it may have many other effects on human minds. However, most or all these features are either arbitrary, or impossible to measure with current technology and philosophical understanding. The main things that we can measure are how much people enjoy it, what it is teaching people, and what kind of social groups form around it.
Therefore, the ideal of art is something that has beneficial effects in all three of these categories. Good art produces pleasure, it teaches things that are true or good, and it unites people around common affiliation with it, ideally in a way that causes good group dynamics and does not cause the group to think less of outsiders. As with most other good things produced by people, we should find ways to produce it as cheaply as possible and in a great variety, and we should encourage people to enjoy whatever they like without feeling a social need to copy the behaviors of other people.
