the same crowd that was into FNaF for the underlying story or lore or w/e would be super-into undertale, cus those would be the same kind of player and both games deliver pretty heavily in that area (undertale definitely more-so)
i don't really tend buy into the whole idea that liking something is a compelling determinant of personality, that always seems like a backwards prejudice to me, but it's probably safe to say that undertale and FNaF share some similarities that would attract the same kinds of players