The answer is that your game prototype should look appropriately good.
Gaming is an experience, and players/publishers can't experience it properly if it looks terrible.
The game should be neat. This means it's simple, clear, and not ugly.
Such a game also appears like a blank canvas to a publisher. It's not so bad it's off-putting, but not so good that you might want to force them to keep your design.
Your layout (of cards and other objects) should just be flat areas of muted colour, with text on top of it. It's simple and functional.
Don't use textures, bevels, gradients, realistic shadows, or other embellishments. The best design is design done by a professional. The second-best design is no design at all, which is what you should do.
In the same way, icons should just be simple, flat-colour shapes. Just go and get free, basic icons from Noun Project. Make better ones later, if you're good with graphic design.
Spend some time trying out a few different fonts. Thoughtful font selection is a very easy way to make your game look good. Do not use standard body fonts.
Go watch a few videos about basic graphic design.
Absolutely do not commission or compose artwork. This is unusual for a prototype. A publisher will want to throw it all away, and assume that you might not agree to that. A design that looks too complete can easily be a negative. This is an extremely common beginner mistake.
Use art from DeviantArt or Google Images. If you're just playtesting, no one will ever know or care that you've used their art as a temporary placeholder.
The new A.I. image generators are great for filling in images you just can't find online. I'd be careful to intentionally create basic images here, not things that look like they should be kept.
At the very least, go and use some free stock photography. Your cards must have artwork. Do not present people with text-only game components. They want to know which card the dragon is, without having to read each card.
The best way to make your artwork look terrible is to use inconsistent styles.
For fantasy or sci-fi, there's enough art available, but for almost anything real-world, I like to use vectors. These are simple, flat-colour illustrations. They give a consistent feel, they're easily editable, and they can be scaled infinitely.
All the above only applies to completed prototypes. For a first prototype, do not spend any more than the absolute minimum amount of time on the layout. It can be hard to resist, I know.
My first prototypes look very basic, and unappealing. They're usually just a bunch of pastel-coloured cards, with a title, some quick text, and primitive artwork.
I don't even bother with proper names. Cards can be called "big guy", "house" etc.
Cards do actually need some artwork, so the playtester can distinguish between them, so I've taken to using huge icons (from the website above) as the card art.
If a game survives its first playtest, I begin gradually upgrading the visual appearance, with each revision. The game ends up neat and presentable, with nice icons, appropriate art, and good typography.