how did Napoleon help european nationalism?

He destroyed what remained of the Holy Roman Empire, crushed Austria. After Napoleon himself was crushed in 1815 the European powers couldn't put it back together again, so they simply created a hodge podge of small German and Italian kingdoms and duchies that eventually became what we know as Germany and Italy.

Smashed the Germanic egg basket and paved the way for later unification.

Created the first Italian monarchy.

By advertising the end of feudal monarchy and feudal ways..