It has nothing to do with racism. This girl's fashion style is "ganguro" (tanned skin, blonde hair, heavy makeup, revealing clothing, etc.). The style is meant to be rebellious towards school regulations and standards and japanese society in general. She's probably portrayed as dumber now that she's become a ganguro bc being studious + smart would go against the idea of rebelling against the school system.
I think the translator meant "ganguro girl" rather than "tanned-skin girl". They probably mixed up "ganguro" (the style) with it's literal translation "tanned-skin."
Σ( ￣□￣||)....is no one gonna? Σ(っ°Д °;)っ