The problem with making a title look like a mobile game is that it could become one.
First of all, the argument that if it looks like a mobile game then it could become a mobile game doesn't hold water for me. Even if it did have a "mobile" artstyle (a claim that I disagree with and will address later), that doesn't mean anything about the gameplay until we actually see the gameplay. If you want to be pessimistic and assume that a simpler artstyle will correlate with simpler gameplay, then that's your perogative but that has no bearing on what will actually happen.
Maybe the gameplay will be simplified. Maybe it won't. But if it is, then that will not have been because of the art style. Plenty of games have progressively simplified mechanics alongside increasingly complex graphics, so there isn't even really a correlation between artstyle and gameplay (these accusations get levelled at series like Starcraft and Fallout all the time).
But I don't see why people even think it "looks like a mobile game." Is it just because it is cartoony? While mobile games frequently have less detailed art, the same could be said for many non-mobile games, even many good ones. I do not think we've seen enough to pass judgement on the artstyle, since, to me, what really defines a "mobile-esque" style is the UI, which is very much absent from current pictures. We haven't seen how the animations are (though I only play Civ with animations disabled), we haven't seen the UI, we haven't seen the leader art. I think that, again with the exceptions of the units (though I do like the boat), the style is very attractive in this one and just says "cartoon" rather than "mobile," much in the same manner games like TF2 or Overwatch do. Obviously, those two are still higher-resolution, but comparing the resolution of models in an FPS to a grand strategy game is pointless.
This is a closeup of a Starcraft II marine. Low-poly, low-resolution textures, wouldn't look out of place in a mobile game. In fact, some mobile games like Deus Ex: The Fall (which was pretty bad but that's beside the point) are far more detailed than that because, like I just said, the strategy genre does not demand high-resolution models. You will rarely zoom in enough to the point where that would even be noticable. From the distance you'll usually be playing Civ at, the only "mobile-esque" quality will be the cartoon artstyle. Both Starcraft II and Civ VI have low-poly models, the major difference being that one has a more "gritty" overarching style and the other looks more like a cartoon. Which is why I personally think that most people who are angry about the new artstyle (not necessarily you guys specifically) are just having a knee-jerk reaction to a significant deviation in style, and not rationally appraising the quality. Sure, it's not as detailed as V, but I've already spent enough time explaining why I don't think that's a bad or even relevant thing.
The majority of cell phone games, ports, etc. are dumbed down casual games with poor DLC models, micro-transactions, and people like Kate Upton doing ridiculous cleavage commercials selling to idiots.
A majority of PC games are crap too, especially what with Greenlight being the cesspool that it is.
I agree that it is just art, but it says things about how the makers view the game.
You're right, it does. And I like what it says. To me, it says that they are focusing on the previously discussed readability and accessibility to new players rather than beautiful environments, which is not what Civ is about. A simplified but still aesthetically pleasing (subjective, of course) style is much more appropriate for a game like Civ.