The miniMAD can playback H264 with 420 pixel-format only (for now).
You can produce your own file using your favorite transcoder (MadMapper uses FFmpeg).
If you want to prevent MadMapper from reducing your movie, there's a dedicated setting in the Advanced tab.
Plus, depending on various parameters, including the output and movie resolutions & framerate, MadMapper can downscale the movie during export in order to have less perceptual degradation while ensuring the smoothest playback possible on the miniMAD.
You can deactivate this feature in the Advanced parameters, but depending on your movie, the result may be less smooth.