Oh, well I don't ship Chasemarsh so I didn't think about that. I think Kate's casting is at least a little complicated because even if the show doesn't mention Kate's race at all, her very existence as a Black victim of bullying automatically adds a racial layer to it. It's another layer that adds to the power imbalance between Kate and Victoria/Nathan. Even if Vic and Nathan aren't depicted as outright racist in the show, that power imbalance is still there, and they're still weaponizing their wealth and white privilege against Kate. This is going to sound controversial and I'm sorry, but I don't think that Kate being Black will make the ship problematic when it wasn't before, so much as it will make the ship more objectively problematic than it already was in the game's canon-- a bully and a victim. Because let's be honest, shippers ignore canon all the time. People ship Grahamscott even though their only interactions are beating each other up, Caulscott even though Nathan is an antagonist, etc.
That said, I know Chasemarsh shippers work within and outside of canon for their ship: an AU where the bullying never happened, for example (the same way people ship Amberfield despite Max and Rachel never meeting in LiS1), or a post-canon world where Victoria found redemption. I'm sure they can do the same for the show characters. But again, I've never interacted with the ship, so my mindset is "ship and let ship."