There's no reason, as shown in the series itself, such as when the gaang is in the EK, that respectful cultural appreciation in fashion can't be done. There's no reason Katara would want to not ever partake in this if married to Zuko, either.

I just really don't think she'd view sometimes engaging in Fire Nation fashion trends as a submission to prior suppression under the past imperial regime or something. There's so much nuance to things and this is one of them I think really goes over antis' heads.