Trashy as in messy, all these characters are messes because of their feelings and the situations they've found or put themselves in;
Yuni has a bunch of trust issues from her home events and neglect from her partner along with mixed feelings for someone who shows actual effort and care for her.
Nanase prioritizes her Ego and ambitions making her naive and a bit narcissistic. She's undermined her relationship about as much as Yuni has sadly. Even though she seems to be trying to work things out between them now, it almost always leans on self pleasing actions from what we've seen. She wants a romantic endeavor with Yuni, only to show how little she actually prioritizes her which is how everything with Fuuko started.
That's why it's a shame that we get a real, genuine moment between Yuni and Nanase for the first time in the series, so much has poisoned things between them at this point that it's only going to get worse the longer they stave off so many complications still sitting with them; they are sitting in a trash heap of their own messes and refuse to clean it.
But considering their age, circumstances, emotions, inexperience with life( and probably hormones ), screwing up a bunch the way they have still makes a lot of sense. Most people don't communicate well or seek guidance when they need those things. The writing makes all of this quite immersive and enjoyable to read, wondering what'll happen.
Personally, I'm still confused Yuni is even giving Nanase the time of day after how upset she seemed with the karaoke scene where both her and Fuuko left and introduced us to glasses girl, and kinda hope Fuuko & Yuni end up together. They feel like they'd grow well together, either enabling their bad traits in each other or fixing them, since they've shown the most genuine connection between any pairings so far after the hotel stay.