There are certainly tons of folks dressing up in historical styles that I do consider to be wearing costumes. Folks who are clearly dressing up in something they are not used to can be considered to be “in costume”. Clothing that is made of the incorrect materials: polyester is the worst, but I consider even modern cottons to be more of a costume, and not really clothing.
I understand that the folks who say: “what a cool costume!” are trying to compliment me, I promise to take it as a compliment as long as I can say in response: “Thanks but it is not a costume. It is clothing.”