Tencent Holdings Ltd.’s deal in June for Supercell Oy valued the Finnish mobile-game maker at more than $10 billion—a valuation that is raising questions in light of a recent revenue setback.

Supercell’s latest game, “Clash Royale,” has delivered declining revenue for four consecutive months, according to SuperData Research Inc. The videogame-research firm estimates July “Clash Royale” revenue at $62 million, less than half of the $133 million the game generated in March, the month it was launched globally with much fanfare....