A Supreme Court ruling adds pressure on the App Store model just as trade tensions weaken the stock.

There is never a great time to lose a Supreme Court hearing but, for Apple Inc., the timing hardly could have been worse.

The U.S. Supreme Court ruled Monday that an antitrust lawsuit against Apple can proceed. The case effectively challenges the company’s business model for its App Store, in which Apple takes a cut ranging from 15% to 30% of the sales and subscriptions generated through apps designed for its devices. The ruling comes as worsening trade tensions with China pose a growing threat to the giant U.S. company that...