does quality of GFX matter ?

[Khephren]

Gameplay is more important, but there is no reason why you can't have good placeholders, that are fast to produce.

you could go the black and white on coloured background like Patapon or insanely twisted shadow planet:

http://thefinalcastle.com/wp-content/plugins/wp-o-matic/cache/3e386_210749-insanely-twisted-shadow-planet-13.jpg
 
http://patapon-hero.webs.com/photos/patapon2.jpg

Or perhaps a fake vector art place holders, they are also fast to do.

Another solution is to go to one of the many sprite banks available online, and use those as your stand in art, you'd get high quality straight away then.

[Khephren]

Use those as placeholders during development, maybe, but don't release with them. Few things inspire less confidence in a game than seeing it assembled out of pre-baked RPG Maker tiles and sprites.

That's why I said placeholders?