A correlation between economic status and disease does not mean that most of the lower class is depressed.
Also depression. It took me one minute to find three sources, could you please make some effort here? I have other things to do.
And you're right, it's not a causative factor necessarily, but I think there's pretty strong evidence at this point, it's hard to find a poor person who doesn't have some condition.
[1] https://www.sciencedirect.com/science/article/abs/pii/S00223...
[2] https://psycnet.apa.org/doiLanding?doi=10.1037/0021-843X.111...
[3] https://ajph.aphapublications.org/doi/full/10.2105/AJPH.93.1...