The print quality is poor, and the image is too light or dark.
You can make prints lighter or darker by adjusting the print density setting in the printer driver dialog, PJ-600 Series Utility or PJ-6xx Utility. 0 is the lightest, 10 is the darkest. The default setting is  in the PJ-623/663/673, and  in the PJ-622/662.
Make sure you are using recommended Brother Genuine Thermal paper.