All 13 witches taught by the sage are cursed in one way or another. The master is cursed with seeing all living things as some kind of Saya no Uta sht. Hence, why the master is so attached to MC - because he's literally the only living thing she can see as, well, a living thing. The monster girl is cursed with being seen as a monster by literally everyone else. Reason why MC has a gun is because he can summon stuff from our world (look at Chapter 2). MC probably has some kind of power to lift the curse, seeing as he isn't affected by two witches' curses so far.
The MC was brought back to life from a corpse that already existed in this world. However he has memories from a world similar to Modern Day. He does not see anything being distorted or monstrous.
Linaria sees other people as monsters. In a flashback, the Luppie's mother (and others) saw her as a monster. The Stuffed Animals saw the princess as a monster. Physalis saw Luppie as a monster. Linaria sees Physalis as a monster. Luppie stated that Physalis and herself were classmates, so I guess that implies that she's also a witch? Linaria also saw the plants as monsterous, but did not see the vase as being distorted, as well as plants in the forest. She did not see buldings being distorted either.
So here's what I've got;
Luppie presumably saw Physalis as a monster, so I assume that all witches see other living things as monstrous distorted versions of what they are. The exception to this is the MC, because he's an undead - so he presumably doesn't fall into the "Living things" category. In Luppies case it seems to be the opposite, she sees normally but everyone else sees her as a monster except the chosen undead, which is what I choose to call the MC from here on.
On the other hand;
In Physalis' explaination of her spells, the people did not look distorted and Luppie's mother presumably raised her to a point if she went to school, which would imply that there was a point where she was not seen as monstrous. The stuffed animals see her as a monster despite not being living things. None of this explains why the Chosen Undead has knowledge of Modern Day despite being a resurrected corpse from the same environment as the witches. Post apocalypse? Maybe but that'd be kind of lame.
Ashen witch... isn't that supposed to be Elaina? The one from the anime of a witch that goes around the world traveling right. Is this manga connected to that story?