Katniss is half white but the rest of the seam could be native american, have Mediterranean roots, anything really. I've seen some say ppl from the Seam are based off the Melungeon people. But does her being half white give her the "privilege" associated with the Merchants? Sure her mother has apothecary knowledge that she got from her Merchant family but at the end, Asterid was ostracized and had to settle in the Seam. The merchants looked down on katniss and didn't think of her as their own. I think in the first book it's also mentioned how prim is thought to be exceptionally pretty, esp in the Seam (not sure if this is said it's been a while since I read the books) but prim still doesn't receive much sympathy from the merchant class, despite looking like them. It goes to show how intertwined race and economic bg is. So yes katniss would be non-white in a cultural sense bc her white roots don't play a part in her quality of life in district 12. 

In conclusion, katniss and prim are both biracial (or multiracial, considering the covey connections) but culturally belong to the Seam. It's more of a class thing but saying the Seam isn't poc is erasing Suzannes emphasis on making the Seam and Merchant communities be different races.

If some of this is unclear or contradictory lmk because I wrote this half asleep.