Given 1: The holiday house is kept completely preserved and stagnant by Carol, especially Dess's room. Preservation of the past is like her whole thing.
Given 2: Carol Does Not Want Susie in her house. Carol especially does not want Susie touching Dess's guitar.
Given 3: Unlike Kris, who has lived in hometown forever, Susie is new in town. As Noelle says in her blog post, she is the 'newest girl'
Conclusion: Carol doesn't want Susie in her house because she's not part of her perfect past: Susie represents change, and the introduction of new things into an established order.
Proposition: What if Carol's attitude towards change being one and the same as her attitude toward Susie also applies to other people?
I mean, to start with the obvious, Noelle has the worlds biggest most nuclear teen crush on Susie. Noelle is also the one that invites Susie into her house, despite the fact she knows her mom won't approve. Noelle wants to go see the lights of the city one day. Noelle wants things to change, just as much as she wants to be with Susie. But when her mother puts her foot down, she's too frightened to advocate for what she wants.
Similarly, Kris is a huge Susie fan. Hell, a most of their independent moments from the player are them being friends with her! And given their duties to the bunker conspiracy involve them being our vessel, we can say that being with Susie incites them to rebellion. They can't help but long for change. They have always been a different element, the only human in town, and the change to that status quo, the introduction of another outsider, helps them be alright with that difference.
Ralsei is shocked by Susie at first, shocked at her lack of adherence to the way he thought things would go, and is compelled beyond careful sweetness and niceness in order to stop her violence. Compelled to be more Ralsei-like, if you will, as when he takes of his cloak and hat. Then, as she shows her gentler sides, she is what makes him realize things about the nature of friendship. She makes him want to go to the festival with her, to want things he "shouldn't". He wants to keep her from knowing the things he knows, that he believes are unchangeable- but when she tells him she's not gonna let that happen, he believes her. Ralsei believes in change.
You can apply this to lots of people; from Berdly's hostility and disbelief that he could ever like Susie, to Toriel welcoming Susie into her home, to Asgore's almost complete lack of acknowledgment of her.
The thing that hits me the most though, is what Susie says in the Hammer of Justice fight, and what she says later. She wishes for eternity, and for tomorrow to be just the same as today. Susie doesn't like change; Susie doesn't like Susie.
