I would say that art is something that is deliberately created to be aesthetically pleasing. If it's done by an AI that was designed with intent to generate such things, I would consider them art, as well.

But if we're talking about definitions, surely what really matters is how most in society understand "art"? Now suppose we went around showing Midjourney-generated pics to random people on the streets and asking them whether it's art or not; how many do you think would say "no", or ask questions about artist's intent before giving an answer?