Send Delta some feedback - maybe along the lines of:
Hello. I am not looking for miles or anything in response to this. When I call Delta I am asked a one question survey - would I hire the person who just helped me? I think that is a great litmus test and I have noticed that the phone service is improving as a result.
I had an odd experience with a FA and thought to myself how I would never allow that person to be my employee. I thought I'd pass that information along to you. Anyone can have an off day and likely I'll never see that particular FA again. But I thought you should know so that you can distinguish your employees having an off day from the ones who seemingly would rather seek other opportunities.