This an odd one.
Much like the answer to true beauty, the answer a lot of people will think is right is that it comes from the inside - it's the gameplay.
Except it kind of isn't. Working in 2D, especially on mobile devices, imposes a limit on what you can and can not do with gameplay; there's only so good you can make interaction on 2 axes. Past a certain point the amount of work needed for a perceivable gain simply isn't worth it in most cases.
Art has far weaker diminishing return, and as 2D games, again especially in the case of mobile development, are inherently simple in regards to mechanics, it becomes incredibly important in differentiation, as many games are similar mechanically.
From my experience the market is cosmetics driven, it's all well in saying "but gamelay is truly important", but once you're at the baseline of mechanical quality, art becomes the most important.
... Well that was depressing