It’s hard to think of another Muppet, or any other fictional character, with so streamlined a motivation as the Cookie Monster. Debuting in the kids’ TV show Sesame Street in 1969, this bug-eyed, blue-furred humanoid has spent almost five decades in the relentless pursuit of sugary biscuits, to the exclusion of all else. Notably, the Cookie Monster never actually eats the cookies he crams so voraciously into the dry, felt cavern of his perpetually hungry mouth. Lacking an esophagus, his tragic fate is to chew, but never swallow. Every bite he takes turns to dust, and then spills from his lips. An avatar of appetite, and perhaps of addiction, the Cookie Monster cannot be satiated. He could munch through every cookie on earth and still never experience the giddy high of a sugar rush. “The Mad Masters,” Stefan Tcherepnin’s solo show at the Stedelijk Museum, appears to ask what would become of this Muppet if he were to attempt to put aside his monomaniacal focus on baked comestibles and search for meaning elsewhere. Entering the gallery space, the viewer encounters four spot-lit sculptural dioramas, in which a series of Cookie Monster–like creatures are displayed in the manner of taxidermied animals in …