i created an account for the sole purpose of replying to this lol :3
i love hatsune miku, but monika could EASILY beat her.
monika would simply delete hatsune miku.
miku, being a vocaloid, is digital, and considering she literally disappeared from the internet for a day (and a song was even made about it), she is basically canonically very vulnerable to being deleted.
monika deletes people. very easily. and monika, being a yandere, will delete anyone who is a threat to her romantic relationship with the player.
now think for a second about how many people have hatsune miku as a waifu....
and also, mesmerizer shows miku will not even try to resist hypnosis, so she probably wont resist monika's control either.
so in other words, in a hatsune miku vs monika fight, miku is COOKED.
monika wins by deleting miku.