Brook managed to injure Big Mom's homies, but not Big Mom herself. Capone and his crew fired rocket launchers at Big Mom and they weren't even able to reach her. Sanji and Luffy squared off against Big Mom at the same time, and ended up getting knocked out off-screen. In an arc with Luffy, Sanji, Jinbe, etc, Nami is the one to deliver the most powerful and effective blow against Big Mom so far.

Zeus is a manifestation of a cloud, but he's also imbued with Big Mom's own soul. The homies in the forest listened to Nami because she had Big Mom's vivre card at first. Now, Big Mom is there herself so the homies no longer listen to Nami, and yet she still managed to win over and attack with Zeus, despite Big Mom and Prometheus both ordering Zeus to come back.

I don't think it was a feat of character development really, it's pretty much in faith with what we expect of Nami and her powers so far through the story. Still, given everyone else's inability to even bother Big Mom a bit, it's definitely impressive that Nami was able to harm her at all.

And I also think you're very much mistaken about Big Mom feeling nothing. Zeus was stuffed full of thunder, he swelled to a huge size before the attack, and the lightning itself was closer to the size of a waterfall than a single lightning bolt. Big Mom herself looked worried when she saw the attack coming, she appeared to be incapacitated once the attack hit her, and the blowout from the attack was powerful enough to rattle the Sunny. I'm not saying it was powerful enough to finish Big Mom off, but I think it's safe to assume that she's injured.