Smiley- the Mutations were those pink/purple dancing triplet monsters with orange hair that appeared on the Muppet Show; they were named as such in the Connie Stevens episode
Philip Kippel-
#2 Connie Stevens- they're Connie's backup group for her number "Teenager in Love". It is in this episode that they are given that name ("Well they aren't the Temptations" "No, those are the Mutations")
#8 Paul Williams- One mutation appears with Sweetums and Thog when Paul is talking about how for the first time he's the tallest person on the show
#14 Sandy Duncan- appear (dressed in gang apparel) in the opening number "A Nice Girl Like Me" with Sandy, Sweetums, Beautiful Day, the Scoffs (vulture-like monsters with long necks) and the Frenchman
#33 Madeline Kahn- dance in Madeline's first number, "Your Feet"
#41 Julie Andrews- appear in closing number, "I Whistle a Happy Tune" with Julie and the monsters
#55 Alice Cooper- one Mutation (apparently female) is in closing number, "School's Out", also in the goodnights
#65 Spike Milligan- appear (as Samurai) in opening number, Fozzie's rendition of "Oklahoma" (in Yokohama)
#66 Leslie Uggams- appear in opening number, "Hey There Good Times" with Leslie and the big monsters
#86 Liza Minnelli- one of the Mutations plays 'Rico' in Liza's number "Copacabana" and appears a couple of times throughout the show and in the goodnights
#93 Doug Henning- two Mutations assist Doug's first act, putting a monster into a box and cutting the box into four
#108 Melissa Manchester- dance in Melissa's closing number, "Your Cheatin' Heart" (set in Spain); two Mutations in the goodnights
Outside of TMS, I don't know. There was one in the opening number, "Hey a Movie" in The Great Muppet Caper (the truckload of monsters that drives by twice). If there are any more Jog might know them.
David "Gorgon Heap" Ebersole