As an owner of a business you have every right to choose how your employees dress and who you serve. If they are instructed to wear uniforms like UPS and FedEX Drivers then by all means they should.If I owned a restaurant, there would be a sign on the door prohibiting pajamas, flip flops and shirts or hats with vulgar expressions on them.
I think the flip flop opinion is kind of on the edge but if it is your business it is your rules. I have walked into many stores and restaurants wearing flip flops in the middle of the summer. Once because I needed to buy spark plugs for the boat and was literally headed to the water, and many other times because it was summer and I was wearing shorts.
Should people have more care in how they dress sure. But let's not act like everyone in the past all wore their uniforms and/or suit and tie. People dressing like slobs have been around forever and I doubt that there has been much of an increase in the number of people publicly dressing like this.
If they show up to work in pajamas and their job is delivering packages to your house by all means you can complain to their employer but if they aren't breaking any company rules and they are ok being that way in public they aren't hurting anybody (except their employer if you stop buying from them because of it). Let's be honest though, the number of people who will stop buying from Amazon because the person delivering their package was in pajamas is probably under 100 due to the shear convenience of ordering something and it showing up a couple days later.