From Zappa Wiki Jawaka
Donovan (Glasgow, May 10, 1946) is a Scottish-British folk singer who rose to fame during the 1960s thanks to songs as "Universal Soldier" (1965), "Mellow Yellow" (1966) and "Atlantis" (1969)
Zappa mentions "Mellow Yellow" in his song Absolutely Free on We're Only In It For The Money (1968), where he talks about the naive dreams of the flower power movement: "The dreams as they live them are all mellow yellow."
The singer is also mentioned in Dental Hygiene Dilemma: "Man, this stuff is great! It's just as if Donovan himself appeared on my very own tv set with words of peace, love and eternal cosmic wisdom. Leading me, guiding me, on paths of everlasting pseudo-karmic negligence, in the very midst of my drug-induced nocturnal emission." Later in this song it turns out that: "that was Billy The Mountain dressed up as Donovan."
Donovan and "Mellow Yellow" are referenced again in We're Turning Again: "They were mellow/They were yellow/They were wearing smelly blankets. They looked like Donovan fans."