People think in reactionary binaries to scenes like this. The point was to show that Toriel, while doting and genuinely caring, may not be this ideal parent it makes it out to seem. Despite it being likely past 12 AM at this point, here she is, possibly drunk, dancing with a random guy in the middle of the house, not even wondering where Kris is despite it being so late and raining outside. In response to Kris and Susie coming home, soaked and probably pretty sullen, she uses a pet name for Susie, encourages Kris to dance too, and basically writes them off by saying there's pancakes in the fridge (leftover from the morning) that they can eat before bed. Then, she proceed to spend the rest of the night making jokes/blaring music with Sans, keeping Kris up.

Toriel isn't a bad person outright and this kind of thing lots of parents do. Parents aren't at their best 24/7; they're able to have bad days, days where they're letting loose, and this sucks even more since Kris comes home from one of the worst days they've had this week. The main issue is that this underlines a lot of other notable issues, like Toriel making zero comment about Kris' room being barren, how she basically doesn't ask how they are directly and instead asks Alphys, and how she doesn't seem to notice the rapid/sudden change in their behaviors.

It shows that Toriel isn't perfect, in the same way that Asgore isn't perfect. That Kris' home isn't some idealized place like Susie initially thought. But, again, Toriel isn't a bad person. She's flawed, but still cares and tries.

People don't seem to understand that someone can be flawed and not a bad person. They saw this moment and either laughed it off, or branded Toriel as a monster. When it's more that she's just neglectful at times and likely doesn't understand how to actually be there for Kris other than dragging them to church, making them food, and hoping things get better. Which, that's a lot of parents.