Why does Sin call Roy "Abercombie"?

She meant it as a slight insult, implying that Roy had changed when he started dating Thea.

Abercombie and Fitch is an upscale clothing brand that is considered "trendy", particularly among the children of middle-to-upper class parents. I'm not sure what the word for "preppy" is these days, but that's how I'd describe it. Roy and Sin, on the other hand, are from the poor part of town, lower-class families, and are likely broke on a regular basis. They would likely consider A&F clothing an expensive and stupid waste of money.

When Roy starts dating Thea, he starts trying to impress her by cleaning up his image a bit. Sin is ribbing him over it by identifying him as "part of the Abercombia and Fitch" crowd. Likely, this is partly Sin teasing her friend over his new relationship, but also partly her sincere commentary on Roy abandoning his roots in an attempt to fit in with his rich girlfriend.

In-universe, Roy is the typical Young "Adult" White Male teen/early 20s pretty boy that Abercrombie & Fitch uses as models and store employees to push their overpriced clothing. Not only does he look the part, he starts dressing like it too. He looks like the poster boy for A&F. Because he is, natch.

Out-of-universe, Abercrombie was likely specifically used because the actor modeled for A&F at 16. He even points out that he finds it funny:

Q: In the preview for the episode, it seems like you're also getting to have some fun with the concept of Roy's hoodies. Is that a piece of clothing you're attached to now, or have you gone hoodie-free because of the part?

A: [Laughs] I've never owned a hoodie, but now it's become a thing. My nephews are wearing red hoodies, and people send me pictures on Twitter of their watch parties where they're wearing red hoodies. It's become a pivotal part of the character. If I do eventually -- some day soon, hopefully -- get to don a costume, I hope a red hoodie is part of it. But it's been fun because now they've cracked jokes about it a few times on the show, and they'll continue to do that -- people calling Roy "Abercrombie" and all that. It's a key part of the character now.

Writers and cast always take the low hanging fruit in making jokes like this.

Could refer to his face being very "Abercombie looking" as Thea refereed to him being when describing her purse snatcher to the authorities.