If you have a nice engine that helps with beautiful lighting / shadows, dynamic range, good animation rigging, or other useful tools related to graphics and visuals, it will be easier for you to make a "beautiful" game with less work than if you have to try and bring some of those things yourself.
It is true that people make stunning games even with janky engines by using great art and design. But the easier it is to implement your vision for graphics, the more likely you'll reach that level.
This is only sort of true. If you load any 3D model into Unreal Engine for example, the lighting and shaders will make it look pretty good without much modification. Godot is getting a big upgrade to its lighting and shader engine in 4.x
It is true that people make stunning games even with janky engines by using great art and design. But the easier it is to implement your vision for graphics, the more likely you'll reach that level.