I see this problem with a lot of the folks using the automatic sprite renderer, but it's not the tool exactly.
The imported textures from ZT2 mods are designed to be seen in higher resolutions so when they're downscaled to ZT1 size you lose a lot of detail. It tries its best to compensate by blending a lot of the colors together into less pixels and that's how you get a blurry image. Your image looks black and white, but it still has blue and red where they were on the actual texture. They're just getting less pixel space and have been 'merged' with surrounding pixels.
You can use Blender compositing nodes to play around with the final render. Box sharpen is a good one to address the blurriness, maybe the RGB tools to mess with the color. It can only get you so far though because again, the original textures imported from Zoo Tycoon 2 were made for higher resolution, and if detail is your main concern then you might need to make your own texture. Simple shapes and colors designed for small sprites usually work best--scaled down you'll see an illusion of detail (which is why I prefer making textures for ZT1 :P). If you've ever seen my hotdog stand, it looks like I added some gradient and texture detail, but it's really just two solid colors. Downscaling and the right lighting did the rest of the work.