Plato and Aristotle, like many philosophers, artists, critics and artlovers since their time, thought that Beauty and Harmony were the main subject of art. But must great art be beautiful? What about a painting like Picasso's Guernica? It is rightly accepted as a great painting. But many who think it is great would not say it is beautiful.
It is actually easy to agree that Guernica is great art, and agree that it is not beautiful, without rejecting beauty as the defining characteristic of art. A piece like Guernica derives its power in part from ugliness and distortion. These qualities are opposites of beauty; so the Beautiful and the Ugly may still be the main features that draw us to such works as this.
Still, this seems like an inadequate response to Guernica. The presence of Beauty (or its opposites) may be important, but that alone does not explain the power of the painting. Picasso painted it after the bombing of the Spanish town of Guernica, perpetrated by Franco's forces during the Spanish civil war. The painting overflows with powerful symbols and references to the horror and senselessness of war. Without these meanings, it would be a far less powerful painting. So while the Beautiful and the Ugly are key characteristics of art, and few works function without some use of them, the Meaningful and the Senseless are equally important characteristics of art, and sometimes they predominate.
(Challenge question: See if you can think of any other categories that play an equally important role in any of the arts.)