We don't know the race or ethnicity of the FA so one cannot infer this was race related. As it's been pointed out how we see the woman dressed in the picture may be different from the way she was dressed in flight. I'm sure I'm showing my age, near 60, but today it seems anything goes anywhere when it comes to attire, including the ACs, FLs, and premium cabins. However, I certainly don't want to go back to the days of suits and ties and dresses. Moreover, I don't think that AA can do anymore than have general comments in regards to appropriate attire on flights. Anything else more gets too far into the weeds.
Years back when I first joined the AC there were some dress standards (no tank tops, flip flops, short shorts) but even that seems to have gone by the wayside.