I have the same experience. I am probably upper middle class (whatever that means) and my kids, as well as my wife like dollar stores. Whenever we visit one I also end up buying at least $10 of random stuff myself (jigsaw puzzles or light sticks or toys to hack).
While I do not dispute the fact that many folks, especially in rural areas are very tight on money, the argument that Dollar Tree cannot be successful without rampant poverty seems unproven.
I think things have spread out enough where we have a true upper middle and upper class now. 750k+ is probably a solid upper class person, and 250K+ is probably solidly "upper-middle".
Maybe you feel middle class among the people that you're friends with, but the facts are that in the US, $85k puts you in the top third and $120k in the top fifth, so $250k which is top 4.3% is far from middle for most definitions of it.
While I do not dispute the fact that many folks, especially in rural areas are very tight on money, the argument that Dollar Tree cannot be successful without rampant poverty seems unproven.
Just a single data point.