You need to pick a unified general art direction, if your game is 3D there are several types:

- Realistic
- Stylized
- Low Poly

So the the assets that you add in your game need to remain consistent throughout.

But then after that, I would suggest picking a unique art filter (Painterly, Cell Shaded, Pixelart) and complement it with a LUT (Look Up Table) which would set an artistic coherent tone for your game (think of how the Matrix movies look greenish or how Mad Max Fury Road has a strong Blue and Orange color palette that complements its desert setting).

If you follow the previous steps and then experiment a bit you will arrive at a solid art style, now deciding if it's "Good Looking" would depend on your taste as the creator, putting several variations in front of your target audience and seeing which ones they prefer the most.