The Japanese version of ‘how cute’. The character Kaguya Shinomiya uses this a lot in the manga called Kaguya Wants to be Confessed to, otherwise known as Kaguya-Sama: Love is War.
It is Shirogane Miyuki’s (another character) least favorite saying of hers.
Yami Kawaii (病みかわいい) is a Japanese aesthetic that was originally born as opposite to the pastel Yume Kawaii aesthetic. The Japanese word "yami" can mean both "dark" or "sick", and refers to its use of dark colors and motifs related to sickness.