Assuming these are slices through an AMR simulation, you could try manually plotting the images using matplotlib's imshow command:
You can see how different interpolation choices look in this example in the matplotlib docs:
This choice to use "interpolation='nearest'" in SlicePlot was intentional and I don't think we want to expose the ability to customize the interpolation, but of course you can create your own visualizations outside of SlicePlot using a FixedResolutionBuffer and the manual plotting interface.