What a lot of the camera-in-my-house squadron tend to not as such notice is that Elly doesn't do so because of the same reason that normal people do. Normal human beings encourage good manners and good English because they care for their children and want them to succeed in life. Elly, on the other hand, is exceedingly superficial, obsessed with how others see her and consumed by the notion that her children are demonic monsters who hate her and get a sadistic thrill when they see her get publicly embarrassed. Given her insane, shortsighted and counterproductive belief that the she's a helpless woman at her last reserve of strength being backed into a corner by the same children who think of themselves as the slaves of a capricious, implacable tyrant, she feels that she has no choice but to resort to the desperate measure of publicly embarrassing them so that they know that she's not totally helpless.
As an example of this premise, the notes for the strip that was republished on 13 January 2012 make it quite clear to me that Lynn seemed to emphasize the importance of good manners not because behaving well in public would help her children later on in life but as a means of displaying her own power over them. Over and again, we see the image of Elly deliberately sabotaging any attempt that her children might make to establish an identity for themselves that wouldn't lead to her sponging off of their hard work later on by somehow embarrassing them and making hypocritical remarks about the need for good manners and good English that she herself is not bound by.
The result of this, of course, is that they end up thinking that speaking correctly and eating like human beings is a form of punishment. We thus have to face the spectacle of an oblivious jerkwad mother making that ridiculous palms-out pleading gesture that no one in the real world makes asking why it is that her children are slobs. It's like how she can't wrap her brain cell around the fact that her witlessly pitting them against one another so that they don't team up to destroy her is why they cannot get along.