What's so bad about her having some fun once in a while?
She's a free woman, she can date strangers if she wants and bring them to her house.
There is nothing wrong with her not being worried about her kid's whereabouts, she's just giving him some freedom.
Why would she worry about giant bloodstain in his room? Kids should have their private space.
She should not have been expected to buy Kris a piano no matter how talented he at it and how important it is to him, because he's adopted anyway.
Toriel literally did nothing wrong and if you disagree, you have mommy issues.