for several reasons:-

1) we couldn't send a nuke big enough to add any material amount of CO2 to the atmosphere of Mars.

2) Instead, we'd need to steer a massive asteroid in the Chixculub class to impact that part of mars, and we don't, of course, have that technology yet

3) the atmosphere of Mars is ALREADY nearly pure CO2, and it is in balance with winter precipitation at the polecaps, so any extra, just becomes extra snow - of course, if you did the whole Chixculub thing, then it would take a few tens of years or even a century or so to stabilise, but it still would.

There's a fundamental difference between the atmospheres of Mars and Earth. Earth's atmosphere only has a trace of CO2, so that trace can be increased or decreased quite easily by various natural and human methods. On Mars, because it already is 97% CO2, you really can't change the system much except by manufacturing exotic and stable greenhouse gasses like SF6.