Contrary to the old saw that “clothes makes the man”, it is attitude that makes the man.
Like it or not, we are visual, and we pick up visual cues very quickly. It is less important what you wear than how you wear it, up to a certain extent.
Depending on your status job-wise in life, it can make a difference with many people. For example, someone in an off-the-rack inexpensive suit from, say, JC Penney’s may have problems being taken seriously, if one is in a job where others are wearing $750+ suits. One can get a pair of causal pants from Target, and they will look like that; someone else who is wearing a $100 pair of casual pants and a $80 shirt is going to look different; and people are going to relate differently to them.
And someone with a pair of $100 designer jeans is going to be out of place with a bunch of guys who are wearing Wranglers.
There is a time and a place for everything, and those who want to paly the “humble” card and dress down where dressing up would not only be situation related but where they are also capable of doing so, may very well be guilty of reverse pride. Pride - the sinful type - comes in many guises.
If you use clothes to put other people down - the “I’m better than you because I clearly have more money and power than you” then it is wrong. (An exception - where you need to be in command of the situation, e.g. as an attorney and you need to have a position of authority). But you also need to be careful of trying to dress “down”, as that can be seen as a reves put-down. Just be yourself; be open and friendly to others. Ultimately we are judged by who we are in relation to others, not how we dressed in relation to them.