Hi Liam,
Inkscape has very limited support for some of the common CAD file interchange formats, and in particular if you’re using DXF, you’ll find the resolution to be disappointing. I’d suggest (as a first approach) importing via EPS, illustrator, or PDF format instead, to see if that makes a difference. Please give that a try and let me know if it makes a difference.
If you’re referring to the TSP art mona lisa example, that’s a very specialized example of raster image conversion — quite a different subject from moving CAD work, which is typically already in vector format. You can make artwork like this by using StippleGen:
http://wiki.evilmadscientist.com/StippleGen