It might make an interesting study to determine why the only martial-arts movies to make major inroads in the American market are those with Muppet spin-offs as heroes. While pondering this question, one can have a reasonably amusing time with this predictable sequel, which is a bit longer on action and shorter on wit and character than the original (hence less good, in my opinion), but still diverting and harmless enough. Although the cast and director are different this timePaige Turco now plays April O’Neil, TV reporter and den mother to the turtles, and Michael Pressman is the directorTodd W. Langen, who collaborated on the script of the original, supplies the same sort of teen patter, and the late Jim Henson’s creature shop is back to provide a couple of drooling beasties. With David Warner and Ernie Reyes Jr. (JR)