Dictionaries associate--with more or less affirmation--that subtext is "the implicit or metaphorical meaning (as of a literary text)".
Can a subtext however apply to meaning conveyed by something other than literature, i.e. through aesthetics?
Yes, it can. There's no reason why a philosophical piece of writing pertaining to aesthetics, for example, can't have a subtext. The same can apply to a conversation; it isn't uncommon to have people speaking about a specific issue but also have other themes involved in their discussion which influence what they say. The easiest example I can think of is two colleagues flirting while talking about work.
The subtext of a work is the work's implied but unspoken ideas. Subtext was originally an acting term.¹ Examples of subtext include subtle social criticisms implied by the text using ambiguous or symbolic language, as well as emotions and desires not in the text that you can read in the actor's face, voice, and gestures.²
The word has since been adopted to mean implied but unspoken ideas in other creative arts. For example, Nijinsky invented a new, "crude" or "brutal" choreography for works such as The Rite Of Spring. It would be correct usage to describe this choreography as a subtext criticizing traditional ballet propriety or social propriety in general.³
Similarly, the Wikipedia article on surrealism⁴ uses the word "subtext" to distinguish Max Ernst's painting Little Machine Constructed by Minimax Dadamax in Person (Von minimax dadamax selbst konstruiertes maschinchen) from his painting The Kiss (Le baiser). The former can be seen as implying an erotic idea (possibly symbolizing the psychological pressures of sexual performance, according to the Guggenheim),⁵ whereas the latter has the erotic idea on the surface of the work.