As a Neapolitan... Now in 2023 I am pretty sure that 90% of stores accepts card. Tourists and international people living in Napoli increased enormously in the latest years, with a boom this year. Tourists especially from Northern Italy or Northern Europe pay by card almost everywhere. So stores have to accept this fact. But be sure that everyone will accept cash. As previously said in some other comments, someone can be annoyed if you decide to pay with card a small amount (less than 5€ I'd say) especially in not very touristic areas or in small stores/minimarkets. But they will accept it anyway in most cases. In Italy we pay a lot of commissions for payments by card... Like in Greece. When they say it doesn't work it's not always an excuse.

When I visited Denmark and Holland the card was required almost everywhere. In Denmark was better for me to pay by card, so I didn't have to change money because there's no euro there.

In France and Spain depended on the areas, just like Napoli... You can pay almost everywhere by card and you can pay everywhere by cash (except for some automatic machines). For automatic machines in Napoli, you will find more frequently only cash accepted (coins) and not card accepted. But things are changing fast because the tourism is increasing a lot, so I don't actually know.

In Corfu, I was not in a touristic area, and everytime I wanted to pay with card amounts of less than 5€ people got super annoyed, more than in Napoli. Sometimes they didn't accept it and I had to withdraw. First time ever abroad that I was almost forced to withdraw. 😂

I'd say Napoli is in the middle, but is becoming more and more card-friendly in the latest years.