Japan just doesn't share these notions. Japan's population is overwhelmingly of one ethnicity. Race isn't anything like as controversial a subject because of this. That's not to say there isn't racism - there's plenty of that, mostly unchecked - but there isn't enough of an ethnically non-Japanese population to raise objections to it. Certainly, notions of affirmative action just aren't part of the mainstream consciousness.
Secondly, it may also be that Iris isn't perceived as being of a different race to the other characters. Aloe, sure, but in anime a dark skinned complexion of the kind Iris has is often simply indicative of an active, outdoorsy lifestyle.
If you feel that this was an undeserved victory for Iris, then fair enough. Personally, I think this was mainly an effort by the writers to present Iris as a more credible trainer, since Ash is likely to be battling her for the Legend Badge at some point in the future. But saying "Iris only won because she's (insert mystery race here)?" There's no basis for that, none at all.
And, at the risk of dredging up an age-old topic: Carole Boston Weatherford had a point.