I like both games but this is subjectively true. I think a lot of people would find the art in Mindustry a lot easier to look at.

Seconding, Mindustry is much more visually pleasing to me than Factorio. From the screenshots I'm looking at, Factorio's graphics just don't have consistent composition, so elements in the same image look out of place. Shadows aren't even going in the same direction or logical lengths, and only sometimes they're pure black giving weirdly high contrast in certain objects and not others. Many environments are various shades of puke colors. The perspective looks weird to me, as if we could turn the map 90 degrees and then all the buildings would look like the leaning tower of Piza.

I would compare and contrast between the original Fallout, perhaps, or as Captain Aggravated here else said, "Factorio does look like Age of Empires with a 3 pack a day habit.".

Now, whether these are problems or style is a matter of opinion, and furthermore whether it should have an appealing style (as Cpt. Agg also said, pollution is a theme in the game) but some of those points are objectively straying from conventionally appealing elements.