The brighter the ground glass, the less sharp the image will be. Usually, a rougher grind will make the image brighter, but a smoother grind will make the image sharper.
The Graflex is a special case, however. Their standard ground glass is pretty rough, but the glass itself also tends to not be very transparent. As a result, they're almost always really dark, and usually not super sharp. Pretty much any replacement, even one you grind yourself with home repair store plate glass, should be a vast improvement (ground to around 800 grit). I actually reground the glass on my Speed Graphic, and it helped, but it's still not as good as some of my other view cameras.
Are you using wide angle lenses? If so, you might consider a Fresnel lens to place over the ground glass. You can DIY one out of a magnifying sheet from an office supply store. They can help out with wide angle lenses being too dark. But if longer lenses are also too dark, then you'll need a new ground glass or a better dark cloth.