Is it okay to wear a rosary as a necklace if you are wearing it to make a fashion statement?
A rosary is a sacramental. According to the 1983 Code of Canon Law, sacramentals should be treated reverently:
Sacred objects, set aside for divine worship by dedication or blessing, are to be treated with reverence. They are not to be made over to secular or inappropriate use, even though they may belong to private persons (canon 1171).
Generally speaking, it would not violate this canon to wear a rosary as a necklace if a person had just cause for doing so. Examples would include keeping the rosary handy for praying throughout the day, preventing loss, or wearing the rosary as a faith statement. Wearing a rosary solely as a fashion accessory, on the other hand, would appear to fall under the canon’s proscription against making over a sacramental into a secular object intended for purely secular use.