The effectiveness and quality of AI art has skyrocketed over the past year or so; it's now easy to generate free pieces of artwork using just a few keywords. However, concerns have been raised just as quickly. For starters, many are worried that companies will see an opportunity to cut costs and bypass actual artists in favor of low-effort, AI-made alternatives. (Basically, it's about professionals losing their livelihoods.) What's more, AI art works by trawling through and using a database of existing artwork to create its own illustrations. Because the original artists aren't credited or compensated, there's an argument that their illustrations are being stolen and/or Frankenstiened into something new without their consent.
That's the crux of this current issue surrounding Terraforming Mars. Players and companies like Coyote and Crow have raised the concerns listed above, along with an observation that AI art is still being used despite the project being so wildly successful (the implication is that the developers have enough money to pay actual people for illustrations or designs rather than using AI).