There are definitely cultural differences at play. I come from an Eastern European country and was brought up to believe that one had to dress "nicely" when leaving the house. That meant no workout gear, loungewear, or PJs, and most definitely nothing stained or dirty. Living in the States now, I do find myself overdressed for my environment 99% of the time. I have tried dressing to fit in, but have realized that it just does not feel comfortable or like me, to me. I do perceive people dressed slovenly as less competent and not professional - of course, some may and will disagree. And of course, some people do prove me wrong, but rarely.
Taking care of your clothes and appearance has absolutely nothing to do with income, but a lot to do with habits and self-respect, in my opinion. Our family had little money, but my parents taught me how to mend clothes, sew on buttons, and do laundry to maximize the wear of my clothes. I also do know many people with very limited incomes who take great care with their appearance and always look polished and nice, and some very wealthy people who always look like they've been living in their clothes for weeks.
Now that I work from home, and having been unemployed for several months prior to that, dressing up has become even more important to me. On the days when I did not take the time to get dressed nicely, I would find myself slipping into a very dark place mentally. I guess it does stem from the fact that I do sincerely believe that clothes make the man, and lolling about the house in PJs or sweats all day negatively affected my self esteem, which in turn made looking for work and forcing myself to get out of bed that much harder.
ETA: I think some people are conflating looking fashionable, trendy, or whatever, with not looking sloppy. I did not think the article was about fashion or expensive clothes, but rather about dressing to look like you've made an effort, which is possible at all income levels.