"Best costume" refers to the outfit or attire that is considered the most impressive, creative, or well-designed among a group of people. Full definition
The film itself rightfully received nominations for best costume design and best cinematography. (filmnoirblonde.com)
Those wins were mainly for technical «below - the - line» achievements — including best costume design, hair and make - up and production design. (timeout.com)
There are more photos at the link showing how well this costume stands out in the real world. (diyhalloweencrafts.com)