My mom loved Fraggle Rock too, and I remember she always cracked up laughing at a particular scene in "Uncle Matt Comes Home." When Gobo and Uncle Matt are on an expedition to find the Crystal Caverns, they get into an argument, and Gobo says "You couldn't lead your way to that rock!" Uncle Matt walks over to the rock, and while pointing at it repeatedly, replies "What, this rock? This rock here? This rock, Gobo?" He sounds so serious, which was what Mom loved about that scene.
One moment that always cracked me up was in "Space Frog Follies." When everyone is singing the "Insect Rock" song and dancing around, Uncle Matt returns from Outer Space with a jar of flies, and he goes up to Marlin, who is banging his head against a rock as part of his dance. It looks painful, but he's obviously having fun.