There is no way Juju Bubble and Jangling Automaton are the worst J cards. At least you can combo the Bubble and Automaton to make them useable. I mean, throw a Lure and some Venom on the Automaton, and you can kill all of their creatures. And the Bubble can at least gain life.
For totally useless cards, you need to look at Jade Monolith and Jandor's Saddlebags. I happen to think Jabari's Influence is also really bad for a J card.