I think the biggest mistake here is the limitations the author put on Miruko's abilities, likely for the sake of being more palatable and for a "fluffy" type romance.
Her being a monster means very little when her only apparent power is "having a scary mouth and scaring people." The mouth is a fairly major detail in deciding if someone will pick this story up from the shelf or pay for a chapter on an online site...it doesn't make Miruko particularly easier to sell, especially if this is not going to go the hentai/smut route. The originally lore says that slit-mouthed women would kill people who answered "no" to "Am I beautiful"...What happened to that particular piece of information?
The idea that the issue is that there aren't enough "rumors" about slit-mouthed women also seems laughably easy to solve...If you scare enough small children, surely you'd see traction. At least make her look legitimately frightening in those moments she tries to scare him, it just underplays her resolution when she still looks adorable to both the reader and the guy. What, she's going to try to scare a guy by just showing her mouth but still having perfect hair, no blood on her face, clothes perfectly intact..? She just seems lazy instead of the dumb airheaded female character she was intended to be at this point.