Should you solid fill a low cinder block wall?
The block will be much more durable and solid if the webs (cavities) are filled with concrete. I'm assuming the blocks are being set in mortar? and not dry set? Mortar can be used as a fill and since the height isn't much you shouldn't see any compromise in strength.   You can float the voids level, but a neater look would be to cap the top blocks with an appropriate and preferred material such as granite.