These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
It's doubtful that they were referring to Ganguro style. Ganguro was a counterculture style born against typical beauty standards that you'd expect of a Yamato Nadeshiko - of black hair, with their bright, usually bleached, hair, and pale skin, with their artificially tanned, stereotypically orange-ish skin - though it literally translates to "black-face," Ganguro isn't based off of mocking black people, and wouldn't be racist. Thing is, the fashion trend died off in Japan within the decade it was born. What doesn't help is that some translations have the girls referring to the women they're basing it all off of as "sisters," and show the women as curvy women with afros. By and large, even tossing away the fact that it was a style from a decade ago, these weren't part of the mainstream Ganguro style. And even with the types of Ganguro that did use afros and browned skin, those are still blackface. Regardless of the author's intention, the effect is that it's seen as blackface. If it was intentionally written in, it's messed up, but at least Riko smacked her for it.