According to Andrew Powell-Morse at Seatsmart, the reading level of the top-selling chart hits has been getting lower and lower for the past 10 years. The average reading level for the lyrics of a chartbuster in 2005 was between the third and fourth grade. The average level last year was between the second and third grade, and a number of the major hits are well below that.
The reading level algorithm takes into account sentence length, word length, number of syllables and so on. This right away can show what one of the problems might be in assessing whether or not a song is really dumb. A song that repeated the word "Mississippi," for example, could score higher than a song that repeated "Utah" just as often. The two would be equally meaningless, though, and could be described fairly as equally stupid.
Powell-Morse notes it's just a fun exercise, which he pretty much has to say because his research shows that the "smartest" lyrics for any rock performer who charted between 2005 and 2014 were written by one Chad Kroeger and company -- the hated Nickelback.
On the other hand, the same research shows that the dumbest lyrics, reading-level wise, come from Ke$ha, and that is most certainly true.