In American English it's common to start that kind of response with "It's funny, [...]". For example, if someone told you they found a hundred dollars on the street last week, you might say, "It's funny, me too!" A few other ways of opening stories are "This one time, ..." or "I remember this once, ..." These are all very colloquial examples but seem appropriate for the situation you described.
It's good-natured humor. The signs are designed to be amusing.
Smart business owners do not purposefully insult their customer base, so we can cross insulting and disdainful off your list:
Sarcastic, insulting, humorous, witty, disdainful?
Oftentimes, the signs allude to certain cultural references, which is why the full extent of the humor might be lost on the learner. For example, there is a famous bumper sticker that says:
This has been around for decades, and it was designed to be a witty way of discouraging people from tailgating you on the road (i.e., from following you too closely). After a while, though, these bumper stickers became more commonplace. As is the case with many witticisms, the joke got stale as it became more ubiquitous. So, variations started to appear:
So, in the case of the sign at the restaurant:
If you are reading this, it means you are literate. Throw waste plates in a dustbin.
the first part of that is just another spinoff of the dozens of bumper stickers that begin with IF YOU CAN READ THIS.
As for the surveillance sign, in the early days of television, there was a television show called Candid Camera, where people would play stunts on unsuspecting victims. When it was time to let the person know they were part of a gag, they would be told the show's famous tagline, "Smile! You're on Candid Camera." Many people in the U.S. are familiar with the tagline, even if they've rarely or never watched the show. So the surveillance sign is playing off that television show and its famous line when the truth is revealed.
If someone is not familiar with these cultural references, then the humor of the variants could easily be lost.
Best Answer
You are looking for the word "Lather", used as a verb. Often in spoken AmE "up" is included after.
If you are talking specifically about soaping someone's hair you would use, "Shampoo".