So I can't really blame a company for taking over a market underserved. They are a symptom of larger changes in the economy and society, that which one SHOULD criticize.
The problem is poor and low to middle income folks everywhere in America seem to be getting poorer.
The Dollar General was a real game changer. Prior to its arrival, you had a 25+ minute/mile (your pick - mostly 55 mph speed limit country roads) drive to the nearest thing passing as a grocery store. When I was a kid there was a gas station, which did sell some food (maybe bread and milk and some canned goods, along with the typical junk food).
DG is such a real improvement for people out that way. It sells the stuff they need at prices that aren't ridiculous.
Crucially, it doesn't displace or undercut anything. There wasn't any other store to displace - even that old gas station is still there! Dollar General is effectively an oasis in a food desert, even with its lack of fresh produce.
I don't know what percentage of Dollar General / Family Dollar / etc type stores are in areas like that, versus in small towns where they might be competing with an IGA or something. But there are definitely places where their presence is a real win.
That is the problem that people have with dollar stores. They have a cheaper cost of entry, but are more expensive overall. Those of us that aren't living paycheck to paycheck have the liberty of buying things a year in advance to get a better price, and Amazon is great for us. But most of the country isn't doing quite so well.