the tsar bomba had a blast radius of about 35 km, the fireball at the center of the explosion was about 3 km in radius
I tend to think Iron Man's propulsion systems can probably move faster than anything we equip our ICBMs with, and his sensors could easily pick them up. I can't fathom the situation where we are creating a large enough nuclear blast that he couldn't outrun, especially considering he could just fly up.
All things being equal, if we did use enough nukes to defeat him, there would hardly be a RL world left to be considered the winner