It does, I think it's just loosely translated to Sir/Madam because you don't really call people Customer.
If you are using them as pronouns, then sir is formal and mister is informal. For example, "Hello, Sir" and "Hey, Mister!" are not the same formality. And, 손님 has the honorific particle -님, so it's formal, not informal.
Mister (Mr.) can only come before a name, like "Mr. Smith." Sir replaces a name entirely.
I just added mister as an accepted translation, but as LinguistSherry said, I would say it isn't a good translation.
No, I think Mister is 아지씨 ((idk how to write it I just learned it by watching Kdramas, that's how they pronounce it) U can say Mister to some random dude on the street, but you'd call...idk...the president Sir