groovy singing hippies
I guess their musical styles and their dress do clash a little. Jeff Moss (who wrote the first bunch of songs like Mad! and Sad and I'm Proud) and Christopher Cerf were very much influenced by such 50's artists as Fats Domino, Jerry Lee Lewis, and Chuck Berry. Jim Henson tired to make the Muppets look current (for 1970), hence the hippie garb.
But those Monotone puppets also sang a version of Spinning Wheel, which in 1969 was a big hit for Blood, Sweat, and Tears. And Jim performed the Little Jerry puppet, not Jerry Nelson. It was actually really cool.
Bob helped put the finishing touches on them (he puts a hat on the Fat Blue puppet) and walks off, the music cues up, and they start WAILING! And on the screen behind them, there's actually spinning wheel/psychedelic light show effect. Very trendy for the time.
You would have to be my age (at least 35) to even vaguely remember this one. hopefully when the promised Sesame box set comes out in October, they can dig up this one.