I don't know the percentage because most of my knowledge of it is anecdotal from myself and people we know. I don't think we can say that one of the biggest problems isn't people's wages being high enough when wages are essentially at 1974 levels still whereas costs and such are at 2018 levels. The problems are very much multi-faceted and not all caused by just that. Student loans as previously discussed here, predatory lending, lack of quality high school education about life skills and critical thinking, healthcare costs, economic policies weighted against lower income people, plain stupidity and greed in some cases, tie-ins with the justice system ruining people's lives (war on drugs type stuff), and on and on.
Of course there are a lot of people that aren't financially responsible. But there's tons of people that still haven't recovered fully from the last recession (we're in this boat), tons that have been devastated by health care costs/issues and tons that are essentially treading water at best and unable to save any money - a lot of different studies are showing this last one. I don't think we can say that a majority of those people are living above their means (unless we start seeing more studies actually focusing on that) but many of them are living below what they should be in "the greatest country on Earth."