Let me circle back around to my original point: this person's argument was that the song, and by extension the score was bad, by definition, because of slant rhymes. That was their whole argument.
My argument is that, objectively, that is not true because there are a ton of critically acclaimed, beloved songs that use slant rhymes. Slant rhymes alone do not invalidate the quality of a song. Just off the top of my head here are some non-rap slant rhymes:
From last season's best song, Omar Sharif (came / waves): "The ship from Egypt always came, sailing in on radio waves"
From the Greatest Showman, a triple slant rhyme (dark / parts / scars): "I am not a stranger to the dark; Hide away, they say, cause we don't want your broken parts; I've learned to be ashamed of all my scars."
There are dozens of these in almost every show. If you're a lyrical purist, that's fine, but then your argument isn't so much "Dead Mom is a bad song" but rather "I hate all modern musicals", which, you know, I can understand.