These images are harmful and do affect people. These images reduce an entire race into something subhuman. These images promotes racism; these images promotes the idea that we are nothing but a costume. Warbonnets are sacred; perhaps you do not understand the meaning of sacred.
The amount that an object is sacred lies within the minds of people, dear. Not within the object itself. If somebody wants to wear a costume as such, you are not the person to tell them not to do it. It isn’t racist to dress as another culture. It isn’t a form of discrimination due to a person’s race. It is nothing more than the interest towards the way an object looks.
It is racist to steal sacred items from a cultural that isn’t your own and it is racist to wear a costume to depict an entire race. Racist stereotypes are racist. And desecrating sacred items is offensive.