I really don’t understand what’s supposed to be so creepy about this. They just like matching outfits as a show of affection. It’s something they mutually enjoy doing and it hurts no one.
Now, if Yuka actually was just copying Hiromi without her consent, as the other girls seem to think is happening, then that would be kind of creepy. But that’s very clearly not what’s happening here.
After her outburst, Hiromi seems worried that she’s scared off Yuka and starts trying to distance herself from her preemptively. Yuka doesn’t want their relationship to end on this misunderstanding, and since Hiromi doesn’t send her a morning selfie to coordinate with, she brings all her cardigans to school so she can match whichever Yuka’s wearing on the spot. Again, not particularly creepy, more Rom-Com sweet. Just a cute way of showing visually how she feels about her.