This tool converts MIDIs into WarioWare DIY records which are stored in proprietary format called MIO. All you have to do is upload your MIDI on this page then you can download it. You can import the MIO using WarioWare DIY Editor
midi2mio works best on MIDIs which have 1 track because multiple tracks will sound jarring. You can try editing your converted record in-game if you want to improve it. Try trimming down your MIDI to include only essential tracks before uploading it. The tool doesn't support any kind of percussion and works best on simple MIDIs.