Nope, it's not an acceptable reason to lash out at someone just because one is feeling stressed.
I used to have a friend who would do this to me and I dropped her friendship like a plague. People who took their stress out on others really ought to learn some self-control and not be led by their emotions all the time. Everybody already has their own set of problems in their lives that makes them feel stressed, and it's just not fair to them to add to their problems by being rude toward them.
If a person has to lash out at someone when they are feeling stressed, it actually speaks volumes about their character, that they are impulsive and they lacked self-control.
What if one day, they get into an argument with someone, and they suddenly have this urge to kill the other person, would they just go along with this urge of theirs?
Ah, customer service, such wonderful memories of being harassed by older women, dealing with incompetent staff and electronic systems and yet still smiling at every new person despite it all.
Is it excusable to be extremely rude? No, people aren't your punching bags.
I used to work in customer service jobs, and those jobs seriously sucks. I have anxiety myself and I have to deal with customers yelling at me. And of course my anxiety gave in and I started giving what the customer wants instead of what my boss wants, and my boss decides to fire me because of this reason.
Never once was I rude to those customers, but I still got fired anyway. But I agree with you that it's not excusable to be rude to people and that people aren't our punching bags.