1. What does it mean?
꾸꾸꾸 repeats 꾸 from 꾸미다, to dress or decorate, for a look that is deliberately and visibly styled. Think coordinated clothes, accessories, and hair that announce you made an effort, in contrast with the apparently effortless 꾸안꾸.
Also written or said as kkukkukku
2. The nuance
It can be enthusiastic or teasing. It describes the amount of visible styling, not whether an outfit is objectively tasteful, expensive, or suitable.
3. Examples
“오늘은 사진 찍는 날이라 꾸꾸꾸로 입었어.”
It's photo day, so I went for a fully styled outfit.
“평소엔 편하게 입지만 공연 갈 때는 꾸꾸꾸가 좋아.”
I usually dress comfortably, but I like dressing all the way up for a concert.
Sources
References support the meaning and usage. Example sentences are written for NoCapWiki.