Looking at the chapter I can understand why people are disappointed with Dragon. Fisher Tiger and Sabo were able to invade and save slaves before, and Kuma has the best fruit to a simple invade and rescue operation. While a frontal assault is suicide, a rescue mission with Kuma should have good odds of succeeding if they knew where exactly she was.
In this way, Dragon only had to learn where she is (even the general location) and tell Kuma. He was a marine so he should have some contacts. At worst he could have begged Garp to tell him where she was (Garp was almost a admiral and was not portrayed as a bad guy). I can understand why someone like Kuma wouldn't know, but just based on background Dragon should be able to learn where the prisoner had been taken (since they even knew she turned into the wife of a CD instead of being sent to ID).
Being absolute pragmatic, there's a possible theory for why he didn't: Kuma was too valuable. We saw him single handled doing a army's work. Even after he took Bonnie he still was doing great work and coming back for dinner with his kid. In this way, while Kuma could possible save someone or even escape if things turned dire, the risk of him being captured and killed or sent to ID was too great. Losing Ginny was not that crippling to the revs but losing Kuma would put their efforts behind in years. So, in this theory, it was not like he couldn't discover where Ginny was or something like that. He just decided to make the necessary sacrifices to the ''cause'' like many revolutionaries in real world did.
It will probably never be confirmed but is a possible theory that would fix this plot hole (not necessarily make Dragon better as a character).