Agree with you OP.
Bad table manners are always noticed and usually (and rightly really) judged. You’re right when you highlight the greediness of it as unappealing, but also bad table manners are something that everyone else is forced to see and cope with. And, honestly, watching someone eat badly often ruins your own desire to eat. Mouths open, loud chewing noises, picking things up from a shared plate and then putting them back(!!!), reaching over the table and over people, pulling bits of half chewed food out of your mouth, putting bones and such on the table rather than on your plate, and many other things: it’s all horrid.
And absolutely people judge others for them.
I was taught that manners aren’t about being an indicator of coming from a certain class or being well educated or any of that (which all are of course true even if not admitted), but it’s about being kind. Manners aren’t a set of arbitrary rules made up just to set us apart, they are there as a guide to being aware of other people. Eating well is about not putting others off their food (or taking too much or whatever), holding doors for others, asking how people are, all acts of kindness that take no effort but can have a great impact on those around you.
It’s like not stopping randomly in the street or at the bottom of the stairs in a tube station, it’s about awareness of others. Of respecting the needs and feelings of strangers (and friends) and of being an aware and caring member of society. Vital in an ever increasingly selfish world.
I’ve had my entire mood changed sometimes during the day by good manners and bad. A smile or a kind gesture can make you feel better about your day.
But why do you think your son is like this? What happened? The over eating might suggest he’s not happy. And of course depression leads to a lack of self care and the slipping of manners too might indicate as much.