I'm running into the same issue as Christopher, but I cannot fix it manually as he did because ST2U resets all of my materials back to default every time I reload Unity. I'm attempting to set up a custom importer as you suggested, but so far as I can tell, tilemap renderers cannot be found via getcomponent. How would you suggest changing the material of each tilemap renderer? Or even just a way to make ST2U stop changing my materials would be very helpful! Thanks!
You know, its the strangest thing. I made a custom importer but had literally no code in the importer. But when I changed the tmx files to use the (empty) custom importer, they all changed over to the sprite-lit-default for whatever reason. I can't explain it but as long as it continues working I'm content with it!
BTW, in the Project Settings for SuperTiled2Unity you can assign the default material that you want all your Tilemap and Sprite renderers to use so for most people this will be the easiest/best way to use a custom material.
If you want sometime a bit more complicated like, say, different materials for different Tiled layers then you'll want to use a custom importer.