Yes—an 18% gray card is traditionally intended as an exposure reference because camera reflected meters are calibrated around a midtone. That mid-gray level makes it useful for judging exposure without the clipping risks of very bright white or very dark black targets.

It can also be used for white balance, but the 18% reflectance itself is not what matters for WB. For white balance, the important property is that the target is neutral in color. A neutral white, light gray, dark gray, or a dedicated color chart can work, as long as it isn’t clipped and is lit by the same light as your subject.

So:

• 18% gray: mainly chosen for exposure metering
• Neutral color: what matters for white balance

For exposure, a gray card is useful, though an incident light meter may be even better. For custom white balance, a neutral reference shot is typically the most effective approach your camera supports. Keep in mind that mixed lighting can still make perfect white balance difficult.