What is the SmoothDisplay (SmoothLineDisplay) setting for both versions? For me, 2012 is OFF by default, 2015 is ON. Using a drawing with contour lines I do see a small difference, much like your images. But setting the smoothlinedisplay to OFF in 2015 then makes it look identical to 2012. Conversely, setting this to ON in 2012 then makes that dwg appear 'fuzzy'. These settings are in 3dconfig(2012) & Graphicsconfig(2015).