You act like the fight is just against the North. She could have taken the throne and the north would come see about her with the Night King in a few months.
Lets go with another scenario.
Could she not have told Jon, from the beginning, that she will prioritize getting on the throne because that way she would have an even bigger army to face the NK once he comes South?
Jon would have objected because by then everyone in the North would be dead.
So yes, she made the choice to prioritize the North. For whatever reasons she may have, she is helping the North. For that they should be grateful.
Hell I know Cersei is not an example to follow for anything, but she is proof that Dany could have made different/selfish choices altogether.