Mapping matters in making your game look prettier and well constructed. Really pretty maps pop out and get a lot of attention. I do find though that the kinds of especially flashy and poppy maps that do get my attention might not necessarily play the best, because there's a lot of visual information for the player to take in, and they may look cluttered or have too many effects going on. This isn't always true, and I'm not claiming it is, but I'm a firm believer in general mapping simplicity. Games like Pokemon, for example, have extremely simple, boring maps but they serve their purpose and play very well.
Good art, to me, can take the place of good mapping for the "pop-out" factor that they cause, and serve equally well for their "advertising" ability. This doesn't make mapping obsolete, I can't remember which Castlevania game it was (it was on the XBox), but I remember playing a game only to feel like movement was a chore due to it's level design, and to find that it was reviewed similarly. It CAN break a game, but as long as the fundamentals are down it's not going to be as large a risk as horrible graphics, a convoluted story, bad gameplay, etc.,