Those who do not have the resources to buy something that is beyond their financial means should not buy it. If they are aware of their financial means and still really want a certain item, like an iPhone, for example (it may seem like something modest to many, but here in Brazil it is very expensive), that same person will buy it in installments, and that is okay, they did what they could.
This is not what defines rich or poor, it is more complex. What is not acceptable is for a person to stop buying the basics for their family just to have something material. That is not being poor, it is being stupid.
Those who are responsible and care more about their family than their own wishes wouldn't even go ahead and buy something on installments only because they want it, but can't afford it, because needs are more important than wants. Your wishes may get fulfilled in the future when you are doing well financially, but if you are not in that position right now, just let it go and focus on what's more important. If you buy something on installments, you will have to pay some money every month, and if you are not earning much, you might have to cut down other expenses to pay the monthly installments, and that isn't a good thing.
Materialistic things are for those who can easily afford them, if you don't have a very good source of income, and you can't manage the expenses along with purchasing materialistic things for yourself only because you want it then you will need to make a choice, and as I said, someone responsible wouldn't choose what they want but would go with what's more important at the moment.