I wrote a cross-platform open source texture viewer. It's still WIP, but already supports 2D textures and cubemaps in DDS, KTX and KTX2 formats and normal image files that are supported by stb_image (JPG, PNG, BMP, TGA, ...).
It allows viewing mipmap levels, also next to each other (for 2D textures).
All in a single small (<3MB) executable.
A first release is available (binaries for x86_64 Linux and Windows): https://github.com/DanielGibson/texview/releases/tag/v0.0.1