>>78127903transformation was a standard established by the court, not in statute, as a tool for addressing the first of the four statutory requirements for fair use in 17 USC 107, which i would encourage you to read immediately.
of course, that very same first requirement also does explicitly favor—in statute!—non-commercial uses over commercial uses. so if you're obsessing about whether his use was "transformative" then you're sitting down to have your argument right on top of the single most dangerous of the four statutory requirements if you're trying to claim fair use for monetized karaoke clips.
no, being transformative doesn't automatically confer fair use protection on your work. that is absolutely, objectively false. transformation is a necessary, but not sufficient condition to achieve fair use protection for your derivative work.
this is to say nothing of the incredible mess of copyright interests in those clips. it's not just cover who have a reasonable claim to take it down. cover has a copyright interest in her singing on top of the recording. they have some kind of license agreement to use the recording, which itself is an authorized derivative (usually, but not always) of a commercial recording, which is an authorized derivative of the underlying composition...