>>42493570
It's fit her character forever, so it's been fanon since the beginning. Unlike Celestia, she can choose not to spend her time on statecraft, so she can have hobbies. Artistic hobbies are appropriately thematic: the moon's role as a muse is well-represented in basically all forms of art. Besides painting or writing poetry, she's also often written as a matron of the arts.
As far as canon, she dreamwalks: a necessarily creative endeavor. I don't think Between Dusk and Dawn really leaves any other possible interpretation other than to closely associate her with artistic creativity. She:
- carves a statue
- impossibly plays a banjo
- sings about wanting to read
- goes to see Ponyliacci
- and an art museum
So it seems like the writers were also pretty aware of these being core distinctions for her from Celestia.