Trying to measure the sadness of lyrics with an algo is tantamount to the early scene in "Dead Poets' Society" where the students are supposed to measure the greatness of poetry by plotting it on a chart.
It's demonstrated in the data: one of the most depressing songs of their entire oeuvre is "Fitter, Happier" but because it's completely metaphorical (or sarcastic), the algo rates it as one of the happiest.
This will always be a big problem in AI. Measuring the literal meaning of words is easy. Measuring the semantics and context of words is a completely different matter.