Might as well share my canon on witches being monster girls since we're having a lore argument.
I view magic as a form of energy. As such, whenever you cast a spell, you give off applied energy to do something. I doubt you can perfectly transfer 100% of that energy to what you want it to do. As such, you have residual magical waste in the form of arcane radiation.
Since you're casting, that means you're getting the most radiation. Now, I believe that this radiation gradually monsterizes you if you're a female or incubizes you if you're a male.
As such, if a human girl cast enough spells or use enough magic, she monsterizes into a witch, dark mage, or other magic-based monster. They may still look human, but they'll give off a vibe, aura, or just a field of arcane radiation that makes others know they aren't human anymore.
Of course, that would take a while. A faster way to monsterize would be for her to dump a fuckton of radiation into herself in the form of a self-applied permanency spell that removes a regular human biological function. You want to not need to eat to survive? Boom, you're a witch. You want to not sleep so you can study or shitpost more? Witch. You wish to become immortal? Witch.
Of course, this is just my opinion.