It’s not!
Currency fluctuations don’t allow for global trade.
That’s why after WW2, the Bretton-Woods system was established with fixed exchange rates. After that came to its end, Europe created a new system. Rates were not pegged to gold but to other European currencies (only 2% deviation allowed, 6% for Italy and Britain).
That created a problem for countries that couldn’t keep up with the German economy: Stabilizing the exchange rate was getting very expensive for those countries. Germany started with zero gold reserves after WW2 and now has second place because of this system.
The solution: a common currency. Germany gave up the privilege of getting paid for their strong currency so weak currency countries could stay in the system. A lot of money for an economically integrated Europe. Italy gave up some sovereignty over the money supply and pledged not to spend too much.
Even without the euro, Italy had an “overvalued currency” that held it back because it kept devaluing its currency with no economic growth and, yes, no financial discipline (although, of course, Italians don’t lack discipline). The introduction of the euro relieved Italy of much of the burden.
While I agree that there was a lot of populist rhetoric in the northern countries, pretending that the euro is bad for Italy and good for Germany is also populistic. The reverse is true.
If you mean fixed exchange rates (going back to the 70s) are bad for Italy then one can discuss that. (But there’s a lot of economic literature against that – just imagine California and Kentucky had different and free-floating currencies and how trade would be impaired)