DXT is lossy, and packs images into 4x4 boxes with paletted colors.
Try setting the texture group to “2D Pixels (unfiltered)” and the compression to “UserInterface2D (RGBA)”; you can also set the Mip settings to “From Texture Group”
You can also set these by right-clicking on the texture asset in the content browser; going to Sprite Actions->Apply Paper2D Texture Settings. This can be done en masse to all your sprite textures if needed.
You may have to adjust for a much larger memory footprint from your sprites to maintain the quality you desire. If you need to use compression to meet memory limitations, you can try expanding the Compression rollout and changing the Compression Quality setting.