It shows a lack of effort and when there's forums packed with eager, inexperienced artists willing to help out for next to nothing (if not nothing), there's little reason for it. If you can't be bothered to source original content. don't get upset when people call you out for it, not matter how strong you consider your story/gameplay to be.

Well said Skoardy, I agree completely.

I don't mind simple/default graphics in some games, even down to ASCII pseudo-graphics. Making games "pretty" can use up a huge amount of resources, which could be better spent (IMO) on more content, bug fixes and polish.

The main thing is that the graphics communicate information well, and don't break immersion.

It comes off as amateurish. I can get that you personally are maybe not the greatest artist, that's fine, but like Skoardy says, why not hire someone? You're selling this game for money, or at least that's the goal... charge an extra five bucks for the title, hire an artist, and put out a product that says "I care about this game" rather than "I give up, I hope the other parts of the game make up for this bad part somehow". Shoot for excellence, make the best game you can make, in every way, because I guarantee your competition is already doing that, and they will crush you if you don't keep up.

Especially when things like graphics are so easy to pick out from screenshots and videos, while components like gameplay and story are harder for potential customers to get a feel for before they buy. Video games are a visual medium, you need visuals.

tl;dr: using the default assets makes your game look less like a high quality commercial product and more like a hobby project that you didn't want to invest any money in developing.

The first contact folks have with your project is the visual, often people have made their minds up by the time the video gets half way. If you feel comfortable in having them dismiss your game before scrolling down to the description, use defaults.