It's an interesting question! From my point of view, "devaluing human effort" (from an artistic perspective) doesn't really matter - humans will still be creating new and interesting art. I'm solely concerned about the shift in economic power/leverage, as this is what materially affects artists.

This means that if your robot creates paintings with an output rate comparable to a human artist, I don't really see anything wrong with it. The issue arises once you're surpassing the limits of the individual, as this is where the power starts to shift.

As an aside, I'm still incredibly fascinated by the capabilities and development of current AI systems. We've created almost universal approximators that exhibit complex behavior which was pretty much unthinkable 15-20 years ago (in the sense that it was expected to take much longer to achieve current results). Sadly, like any other invention, this incredible technology is being abused by capitalists and populists for profit and gain at the expense of everyone else.