WMG / Ninja Turtles: The Next Mutation

The turtles and Splinter in this show are faulty clones

The reason why the heroes in Next Mutation are so dumb and juvenile compared to other incarnations of the Ninja Turtles is because they are not the real turtles and Splinter but clones created by the Foot Clan to train their ninjas for future confrontations with the real turtles, but Shredder deemed the clones a failure due to their obvious intellectual deficiency and dumped them in the sewers.