If we take into account how the mirror was the closest thing to the witch that stood out from the eye (the brush is definitely too small to be noticed, and if I saw a mirror lying on the ground in a clearing where a sacrificial ritual is being performed, unless I thought that the witch is quite narcissistic and wants to control her hair all the time, I would immediately think that it has something to do with the ritual itself) and assuming in a somewhat metanarrative thought that Chilo and Nada have explained in broad strokes during the journey the powers of the witch herself and how to possibly beat her - after all, it's not that in a 4-koma you can really explain all the steps by the wire or by the mark, or it would be a series ten times longer - the fact that it's the first thing that Oscar has broken is not so strange and forced.
The only true point to be made is that the mirror, apart from the only time it was shown a few tables ago to show the demonic reflection of the witch was never mentioned, which makes the narrative spectator a bit forced (all we expected was the brush, or maybe the cucumbers, and instead it was a completely unexpected element). Then I honestly didn’t notice if the mirror actually appeared more often and I just didn’t notice...
@Wolfwhistler
maybe just because the witch has performed the ritual a lot of other times with ease gave the positive result for granted, a bit like when you are in the mountains you pay a lot of attention in the most difficult sections, and then stumble and roll in the stretch of flat and smooth road because you no longer pay attention to where you put your feet. Not expecting anyone to attack her, she completely skipped the security of the area.