No, this shouldn't be a factor. When you look in a mirror your eyes focus on the image in the mirror, not on the mirror surface itself. This image is located at an effective distance that's the sum of the distance of your eye from the mirror plus the distance of the object you're looking at to the mirror. So if you're looking at the car that's 50' back your eyes have to focus for that 50' distance no matter whether the mirror is located only a few inches from your eye or several feet away.

What is sometimes an issue is the degree to which one of your eyes is dominant. With a helmet or eyeglass-mounted mirror it's typically only visible by your left eye. If you have strong right-eye dominance you might have trouble concentrating on the image in the mirror seen only by your left eye. Easiest way to check is just to try one out. Bike shops should be willing to let you test one of their mirrors to see how well it works for you.