Saying it's a matter of scale is an understatement. You need mass to keep the core molten, along with the right combination of elements to produce a working magnetic field that doesn't fade over time. Mars has neither of those things going for it.

If you want to terraform Mars, you would have to rebuild it from scratch...basically creating a whole new planet with the right core composition and size. Even if you could pull all that material into one location and guide it all into a stable orbit, it would then take hundreds of millions of years to cool and set into something we could even hope to live on.

I don't think human beings will ever be capable of that level of geoengineering. It's just not realistic given the alternatives.

Even not underestimating the scale, I'm not sure it would work because all the debris would need to be ejected from the thousands-of-kilometers-deep hole. And then you'd also have to have a solution to stop the walls from caving in before the next bomb had a chance to arrive. It's almost as if you not only need thousands of extremely powerful (even for nukes) bombs, but also need to deliver them in a continuous stream to keep the blast pressure up and the hole open.

I feel like, at that point, the easier strategy to accomplish your goal would be redirecting a large asteroid to impact the planet, or something like that.