I pass homeless people and think I would like to give them some money but I don't carry cash any more.
If I saw a person with a QR code I'd assume they are someone who wants to game whatever system (and so has other options besides my donation). If they wanted to deal with a financial institution, they can do all the other BS. I'd rather give money to someone who appears to have rejected that.
If you asked this question sincerely and really want to give money to homeless people you see, carry cash. Easier to make that small change in yourself than try to implement a technical solution for a large and diverse population of people who have obvious financial problems.