On a recent walk down a nearby street, an older man stopped, pointed at the sign pictured below, and said, “It should say: Don’t be a jerk and pick up after your pet.” I made a startled quick response and continued on my walk.

The sign is very polite. It includes both “please” and “thank you.” The politeness is hard to miss in multiple ways: the polite words are at the top and bottom in a different font and the signs are all throughout the neighborhood.
At the same time, the niceties cannot cover up several unpleasant aspects of this sign. The polite words surround a command (“pick up after your pet”). The sign references poop. Finally, the need for the signs suggests not everyone follows these rules.
Would the sign be more effective if it did away with the politeness? Is the potential offender of this request going to be swayed by the politeness? There are other options for the sign. It could include no polite phrases. It could reference consequences, such as fines. It could appeal to shared norms (example: “keep our neighborhood clean”).
The politeness of the sign might be more about the people putting it up and upholding these guidelines. They want to reference a community atmosphere where people collectively care for the environment. Pronouncing a command does not seem to be as bad when couched in polite terms.
The comment of the man who talked to me hints at the ongoing issue at hand: a polite sign may not produce the desired outcome. But, if signs become more pointed or punitive, all semblance of peaceful neighborhood life might disappear.