It becomes cheese only if the enemies are unrealistically permissive of being destroyed by it.

In a tabletop situation, this would be a strong tactic and a good edge to give yourselves, but the creature, being creatures, wouldn't sit there in the fire and die - they'd get out and start looking for the source of the disturbance, so it would probably still end up in a reasonable fight, just one where the party, through a clever opening gambit, had a distinct upper hand.

In the game, where the enemies will just blindly sit about and do nothing until they are killed, because they can't detect player characters from their current situation, then yes - that becomes cheese because it's circumventing game-play and making the game behave in unintended ways to your benefit. Swen would approve, and likely wax poetic about what a great spell flaming sphere is because it lets you cheat the game like that.