Would a nuclear explosion over one of the Earth’s magnetic poles momentarily disrupt/weaken the Earth’s magnetic field?
There are a few different questions lurking here.


No, it would not have any lasting effect on the Earth's magnetic field. That field is not generated at the surface, at the poles or otherwise.
Yes, it would produce an electromagnetic pulse, and that could be harmful to unshielded electronics in at least a line-of-sight sense.
Yes, this EMP would definitely cause localized magnetic effects for a very brief moment, which one could argue constitute a change in the natural magnetic field.
I'm not sure whether there's any reason to think that the earth's magnetic field could help these effects to propagate beyond line of sight - which I think is what you're really asking. Perhaps a physicist could chime in...