Being a clone of Donatello, Dark Donnie is the smartest of all the clones. He doesn't always act it though, being snarky to their leader Dark Leo and fighting for food like the rest. But that doesn't mean he's not intelligent; he is presumed to be the creator of the Terrorpin, their ship that is the equivalent to the Hovershell, and probably all of the other gadgets he is seen using (e.g. the giant magnet and smoke bombs in Bad Blood)