Realize you need several parts to get best use, as the mushroom cloud has a stalk and a fireball, and each has different materials

use metablobbed spheres or tori to build the stalk

the fireball cloud you can make using several tori arranged in a circle and metablobbed

assign a displacement material or volumetrics to them as you see fit.
or, i
nner displaced solid, and surround with a duplicated larger versions with a volumetric for smoke/steam etc.

best way I can show you is to go look for the nuclear fireball from the game FALLOUT3, when they blow up the nuke in Megaton ;)

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