Why a 36-gun vessel is a frigate, not a brig

Could a 36-gun ship plausibly be described as a brig in the early eighteenth century?

No; that armament belongs to a frigate, while a brig would normally carry far fewer guns.

The game's description of a 36-gun vessel as a brig does not fit its armament. Brigs normally carried roughly eighteen guns or fewer, with only unusually large examples approaching twenty-four. A ship carrying thirty-six to forty guns instead belongs among frigates and approaches fifth-rate strength in Royal Navy terms. The distinction separates a dramatic label from the rig, size, and armament categories used in naval history.