It definitely influences purchases.
...but not sales.
I remember as a kid, I would always buy the packs that looked the coolest or had my favorite pokemon. And I imagine other kids still do this. Therefore, the package art influences their purchases. But like omahanime said, all of the cards must be sold eventually, so the sales total remains even from all the packs over time.
But on games, like PMJ said, I think its different. Nintendo is very capable of... say making more FireRed than LeafGreen games if the FireRed sells more (because of Charizard or whatever) - But I'm not sure if they actually would make more Red than Blue or whatever. I'm sure someone can confirm or deny that.