>>716463439For me, it's because my mom struggled with alcoholism for a while.
It brought back some dark feelings of not even knowing who she is any more. Feeling like she was letting this monster take over her on a nightly basis as a way of showing how much she hated the rest of the family.
I've got a normal nuclear family, and even my dad agrees that she was tearing us all apart. He was considering running away to another state without a word at some point. (I was an adult at the time, and my brother was about to be.)
The scene is designed to feel uncomfortable, and make you feel like you're alone in this.
Kris and Susie just went through some harrowing experience, and what they're met with isn't the warm comfort of Toriel's kindness, it's her acting unlike herself, in a loud energetic situation. being shocked when you expect to be able to breathe and relax a bit can exacerbate the feeling of unease greatly.
They seem either oblivious or dismissive of just what a dour mood Kris and Susie are in. and Susie changes her mind about going back to Kris's house, which she was so adamant about a moment ago.
I'm not going to go as far as to say it makes Toriel a bad mother or anything, but it's certainly not portrayed in a flattering way for her.
The point is that the scene isn't about her, it's how Susie and Kris react to it, and how they're feeling.