The art of your game is very important, not quite as important as gameplay, but just as key.
When people play your game, you'll want them to be embraced by the mood. Also, when people check out the page for your newly finished game, you want them to get the right idea. This isn't to say you need to be great at art, some of my favorite games certainly don't look the prettiest- you'll still be able to find an audience.
Anyways, more on the mood part. If your player character is traversing a highly dangerous area, you wouldn't want bright colors and happy themes (That is unless your goal is to divert expectations) . Same works vice versa.
Getting your art style pinned is key. I say "your" art style, but I don't mean you have to be unique. There are of plenty of styles that are well known and have alot of reference for.
For example:
Picking one is important because- well- look at the difference!
Also, remember, you'll have to make EVERYTHING in this style. The fidelity of your art doesn't matter all that much, but consistency does.