Only if people are dumb.
If someone pretends to pay you with by showing fake QR codes, it's very simple to check as the money should be sent instantly to your account. If you don't receive any money transfer, it's fake.
It's also very safe to pay with QR codes in a shop. After you scan the code, you have to approve the transaction on your app with the amount of the purchase.
It's actually much safer than paying with credit cards, as you don't disclose any information such as card number and code. No one can re-use any information to make a fraudulous purchase paid by your bank account.