The remaster buildings and units look more cartoony because they appear over-saturated and shiny and clean, which makes them look artificial and 2D against the more gritty and textured background environments. They should be de-saturated a bit and maybe have some graininess added to them to make them feel more realistically integrated as part of the scene.
Cartoons often go for a simplistic, colourful, soft, non-integrity look. When people perceive similar qualities in other work, it gives an impression of a cartoonish appearance.
I think the C&C remaster buildings, units, and environmental props (like trees) are looking too clean, pristine, shiny, like flat 2D stickers on top of the environment. More realistic shading and taking some of the sheen off the buildings and units, as well as possibly de-saturating buildings and units by a touch, would help significantly to create a more immersive, deep, and C&C-relateable appearance.
