“This is so lame,” moans a spectator at a third-tier circus featuring a bovine funambulist, rocket-propelled hound, juggling ferret, and weightlifting chihuahua. That judgment, uttered early on, might apply to the rest of this flatly animated, padded-out trip to the kiddie-flick doghouse. Clifford (voiced by the late John Ritter) is a gigantic crimson canine who assists the ragtag crew in their quest to win the Tummy Yummies pet food talent contest. As Larry, the kindly animal trainer, conveniently remarks to the aforementioned ferret, “We sure could use a big red dog for our show.” Still, Clifford has its irreducible mysteries. To wit: Why big? Why red? Why dog?