I think it's more like, people who have transitioned from having no money, where money would solve a lot of problems, to having money, where those problems have been replaced with other problems, and they are shocked to discover that having money doesn't eliminate all problems. Even if they have fewer problems than they did when they had no money, their current problems are more frustrating because they expected to have fewer problems with more money.
The hard part is that point moves. I have very few problems. And insurance covers some things (supposedly). But if my spouse died and I suddenly had to throw child care into the mix, and changing my schedule to be the one that takes them to/from school... I could easily wind up where new problems form. I thought I had enough money, but I only had enough money for the specific circumstances
There was a study about that around 15 years ago and the number was $78,000. Money does make you happy and solve problems to an extent, or rather it eliminates miserable situations created by poverty. Beyond that though, it's up to you to find happiness. This was 15 years ago, so that number is probably double now.
Keep striving! You seriously can do it. Keep learning. Keep working. Keep trying, and don't give up. Make a plan, set some goals, write them down, and follow through with it. It shouldn't be as ridiculously fucking grueling hard to claw your way up the latter as it is, but it's worth it when you get there.
Some people stressed about paying rent.
Some people stressed about paying mortgage.
Some people stressed about paying loan on their investment.
Some people stressed about being assassinated on the street.
It’s more of a gratitude and cash flow issue.
You can stopped at any point, but most people just want more.
At least in the context of the song, I'm pretty sure the "mo' problems" part comes from the fact that having "mo' money" makes you a bigger target for people who would want to cut themselves in on your money and/or your sources of money.
They have more problems because they're problematic people, and problematic people with lots of financial resources create tons of problems. Money solves most problems for responsible people. Not all problems obviously, but most.
Also, biggie had to work for his money, deal with the corporate record industry, and lived in a high-crime neighborhood. If you don't have to work, and you can afford to outsource your problems to others, then you can have a lot of money without problems.