It's not impossible to find a few decent "private party" mowing jobs that'll pay a few bucks. If you have enough nerve, you might even get away without carrying liability insurance. But once you start reaching out for bigger jobs, or clients who're dealing with commercial property that may or may not actually belong to them....The rules begin to change. And go after municipal/state/federal work and the rules REALLY change. Steep insurance requirements, performance bonds, 90 days (+) waiting for PARTIAL payment, having enough equipment and enough crew to do the job in a predetermined time frame, and a laundry list of other hoops to jump through. Profit margins grow tight when you're doing that sort of work.
Stay relatively small, work for "beer money", and stay a "one man band", and life is simple. Go big and commercial mowing is absolutely no different than any OTHER business venture.