Doesn't Nami have a soft spot for kids (like at Punk Hazard)? Seems in character for her to say 'that's it!' when Tama gets hurt by Ulti. They've been running long enough, Nami having back-up from Big Mom should just make her even more confident in taking on Ulti. Obviously she likely won't be able to beat Ulti until Zeus shows up but still, I wouldn't say it's out of character for her to dislike people that harm kids, especially one that she's close with. Nami just doesn't appreciate Ulti harming a kid like Tama so she's lashing out, not unusual when she went out of her way to convince Sanji to agree to help those kids at Punk Hazard when she didn't even know them.
Not only Tama is a little girl, she also happens to be the victim of a tyrant that took over her island and made the life of everyone there a living hell and is trying to fight back the way she can. Ring a bell ? Who happened to be in that situation? Geez I wonder why Nami would feel it extra hard'
Nami showing Luffy levels of bullshit and stupid by facing Ulti right here. If she cares about Tama she should stay with her and stopped before she hit Tama not after. Also BM is already there to revenge Tama. Literally no reason for Nami act like that. Pure bullshit proving how much lost Oda is in his own history.
Here Nami losing all inhibition and talking back to a 300 millions bounty enemy with Usopp even telling her to calm down and not do anything stupid. That happened 350 chapters ago.
The point of Nami and Usopp is that they're cowardly characters, but like the rest of the Straw Hats, they're courageous as fuck when pushed to their limits. For Nami, it's children, just like Punk Hazard. For Chopper, it's using medicine to do evil, for Luffy, it's hurting his friends or treating you own underlings like shit. I can go on