Most people who rely on gig work are financially vulnerable, according to findings by the Federal Reserve.

Three in 10 Americans do gig work of some kind, a broad class of temporary employment that covers everything from babysitting and house cleaning to driving for ride-sharing applications such as Uber and Lyft, according to the Fed’s annual report on the financial health of U.S. households.

But those who depend on such work are struggling to make ends meet. Nearly one-fifth of gig workers rely on it for their main source of income, and more than half of these people would have...