No, there's no way to prevent a mirror match.
You could set up your Order in a way that that character NEVER gets chosen as a CPU to be played against (by anyone) and is for playable selection only, but you probably wouldn't want to do that.
Make the character order=9 and have the game end at order=8, for example.
However, MUGEN isn't smart enough to not pit you against the character you chose.