Do you use the staircase method or the flat surface method? Because the staircase method often produces much higher quality images. The flat surface method is usually only good for toony map art. If you don't know what this is, it is placing blocks at different heights, which alters the brightness of a pixel which makes map art higher quality. I might buy a map art if I feel like I have the spare grass.
The staircase method takes around 6 hours per map, this is simply not worth it sadly. Thank you, making more progress you told me in game not to worry about it, sorry.