No, it's a simile. An example of a synechdoche would be "All hands on deck," except in those rare cases when one is playing bridge on the deck of a ship and you want everyone to put all of their cards down.
No, "dumb as a box of rocks" is an example of a simile, not a synecdoche. A synecdoche is a figure of speech where a part of something is used to represent the whole.
Her laughter was like opening a treasure box, full of hidden delights.
The phrase "dumb as a box of rocks" is a colloquial way of saying someone is not intelligent or very foolish. It implies a lack of mental capacity or common sense.
...a box of rocks. ...a bag of hammers. ...a fence post. ...a Dodo bird.
It means really dumb!
No
The oldest simile for 'as dumb as' is 'As dumb as a doornail'. Others include: As dumb as a wet noodle. As dumb as an unpowered computer. As dumb as a library with no books.
No. Box does rhyme with rocks, though.
boulder holder
rocks
because there dumb
boulder holder
The population isn't exactly dumb. It may seem that way if you are very intelligent. Also if you are talking to an un-educated crowd they would seem dumb to you. A lot of things can be blamed for the un-education though: Poor schools, Poor teachers, Poor parents, Non motivated children, Bad neighborhood or bad influence. But these are just excuses really. None of these can be truly called "the reason for un-education".