As for bad VERSIONS of songs, I almost forgot:

I love Stevie Nicks - usually - but her recording of "Sah-lunt Night" is one of the most groan-worthy things I've ever heard come over my radio! Gawdawful!

Now when it comes to really bad Christmas SONGS, I second the nomination of "The Christmas Shoes."
But The Beatles' "So This Is Christmas" is not only one of the worst, most Hallmark-card-ish piece of "lyric" doggerel but pretty danged trite musically as well.
What makes it so bad is that it gets played over and over ad nauseum every year!