I only think headcanons are only valid if there's evidence behind it or their sexuality is never explicitly stated (for example i like the idea that sans and toriel are dating)
a headcanon is only you trying to justify fetishizing a character or person; 😐, i feel like a headcanon is a assumption, if something isn’t actually canon about someone and that you give someone a certain self role that you chose to see them as based off certain characteristics that they show towards people and personality overall.