I guess I might define "guilty pleasure" differently. I don't care much for critics, who I always cared about were peers. I guess as a young teen-aged boy I was insecure and concerned that I'd be ridiculed for my tastes in music. And while most of these artists were respectable they also had their schmaltzy aspects and you could get reprobated for your tastes.
I actually think John Denver wasn't as much of a risk. He got very schmaltzy, but he was associated/wrote a few very good songs. But how many good songs are written by anyone? Rocky Mountain High, Country Roads & Leaving on a Jet Plane are outstanding songs. Look at, say, Laura Nyro and Jimmy Webb. Nyro maybe wrote six really great songs and Webb maybe six to twelve. That just goes to show how writers who have put out dozens of songs are so unusual.
I think The Carpenters were way over exposed, and harmed by their respectability. But yes, ignoring "We've Only Just Begun" - which I blame for an uptick in divorces, Karen sang a lot of great songs, I think "Superstar" is my favorite. I liked the Bee Gees, but there was a perception of them being one step above bubble gum. When we wanted to punish our boys, we'd play the Bee Gees greatest hits for HOURS.