Why does the whole town explode when Misses flies her dragon over it and breathes fire on the STONE buildings (less than a second per STONE house)?
It was previously established that the fire from dragons could destroy stone buildings. Remember Harrenhal?
This was all that was left after Aegon Targaryen and his two sisters attacked Harrenhal during the War of Conquest https://gameofthrones.fandom.com/wiki/Burning_of_Harrenhal
If you want a non-lore reason though, the city was destroyed so quickly to save on CGI budget. It's a common technique in tv shows, for instance in Star Trek Deep Space 9, during the Dominion War, none of the ships are shown to have shields during the battle sequences, allowing them to be destroyed in seconds. This was done to cut down on the budget.