No-Coverage Answer
There is no mention of Kurt Cobain, Nirvana, or any connection between specific musicians and the Karistus in the Taygetan contact material. The concept of "Karistus descent" as applied to human musicians or cultural figures is not discussed.
The Karistus are profiled in dedicated transcripts (232, 234) as a 6D race based around Jupiter, linked to the Christ/Dhor Kristil concept, who deliberately appear with wings and are partially the origin of the Earth angel mythology. They claim all Lyrian races (including humans and Taygetans) are Karistus hybrids at a soul level (234). But no specific connection to individual musicians, artists, or cultural works is made.
See also: What are the Karistus and what are they doing about Earth? (covers the full Karistus profile)
Searched: All 834+ transcripts (grep for "Cobain," "Nirvana," "Kurt"). Zero results.