fyi, there was a POC project called "3DSAmnesic". It uses ffmpeg to decode the mp4 file. It lacks a fancy UI, file selection, and it's discontinued and no longer maintained. I failed to build it, but you can try it by yourself.
In terms of performance, since O3DS is not powerful at all, Framerate drops terribly with h264 encoded videos. N3DS can somewhat play it decently at 30FPS.
So, the reason why there's no decent MP4 player in 3DS, is that this device lacks a computing power to decode the mp4 file using only CPU. As I said, It is nearly impossible to watch h264 encoded videos with O3DS.
3DS seems like it has a hardware decoder, and possibly .moflex formats are getting decoded with it, but no one has found the way to make use of it in homebrew.