When koyukkuri call their dead or mutilated relatives 'monsters', it's fairly common.
Just as often if not more so, I've seen them just go "SHCARY!!!!" And leave off the monster bit. No real need to tag that. Goes with loss of recognition from accessory loss. Mutilate or change a persons appearance enough and even humans don't recognize other humans. We just amp it up to 11 with yukkuri. Most things that need tags, have them after 8+years of the sites existance. Things like eye_loss, accessory_loss, puffing, licking. Actions and obvious stuff. Most phrase tags such as the ancient its_like_flying_in_the_sky are pretty much obsolete or joke tags. Phrases aren't universally used and translations vary. I've been around long enough that I think that's why we don't tag we them. And the ones that are, take it easy, far too many to tag that kind of thing.
Also, I'm still not sure it's her mom she's calling scary. Could just as well be the Anon. Though you might be right.
eh, it's not common at all and we don't really want to start tagging things based on what they say. Plus "monster" is not a descriptive/informative tag and shouldn't be used anyway.
Yunyaaaaaaaa!!Djie eajy!Mwommie where are yuuuuuu!?
Quuckly shabe RemiyuuShcary monshter!
Don'ch comeeee!Yu... little... one... run...