That doesn't mean that you don't ever need prep, or that anything beyond minimal prep is always bad. But the idea that spending excessive time on extraneous stuff like art somehow makes for a better game flies in the face of all of my experience.
I agree with the others that this is simply "flies in face" of all of
YOUR experience.
Imagination is great (obviously) but having a shared vision given by art creates a more unified concept of what is going on in the game.
It is like the difference between Theatre of Mind combat and using battle maps (or sketches, minis, tokens, whatever). Many groups play just fine with ToM combat (I use it myself sometimes still), but a lot of players (newer players especially IME) prefer some form of battle map to help them visualize the scene, terrain, develop tactics, etc.
Neither is superior other than what works best for a particular group. As kids, we did nearly alway ToM with occassional map sketches. Now, 90% of my encounters (even random ones) use battle maps because it works best for my players.
Physical props, like the riddle scroll
@ECMO3 mentions above is another example of how "art" can be more than just window-dressing.
Prepping all of that takes time, which I am happy to do to create a more enjoyable experience for the group as a whole.
I'll add that sometimes unexpected things happen, the PCs take a different path or whatever, so I have to adlib and wing it, in which case I'll resort to ToM so I don't waste game time in "further prep" and my group is fine with that when it happens, but it isn't their preference.