Johnny Rogan in The Complete Guide to the Music of Neil Young observes that of all the songs on Tonight's The Night, "Borrowed Tune" is the one that has Young singing "at his most wasted." That's quite an assessment considering the entire album is one gaping wound of musical agony, mourning, and resentment.
Tonight's The Night is a full set of Young openly grieving the drug-related deaths of roadie Bruce Berry and guitarist Danny Whitten. In "Borrowed Tune," he's so crushed, drunk, and stoned that he comes right out and admits that the song uses a Rolling Stone tune.
The rest of the song's lyrics reveal Young confronting mortality and questions of nihilism. "I'm climbin' this ladder, my head in the clouds," he sings, presumably referring to his career ambitions. "I hope that it matters, I'm havin' my doubts."
If we all just die anyway, then does anything matter? Why even make music, Young's asking.