Its not just a "tier" of store, its a "genre" of store. Stores like Dollar General are not really grocers, they just happen to carry some food products. They typically do not carry any fresh foods. So its not a matter of their tomatoes are somehow worse quality, its that they do not sell tomatoes. Its not that their meats are worse quality, they do not sell fresh meat. They practically only sell pre-packaged goods. Think about the few isles of junk food and small packages of household products (soaps and what not) you might find at a gas station, and scale that up ~800m^2.

If its not something that is OK to sit on a shelf for a few months, you won't find it at a Dollar General.

When it comes to actual fresh foods (which can be found if you go to actual grocery stores), those are highly regulated. You'll find fancier varieties at fancier grocery stores, but in the end a yellow onion at Kroger is about the same as a yellow onion in Safeway or Publix or Albertsons or HEB or Whole Foods.