According to §30.4.1.2/2:
The mutex types shall meet the Lockable requirements (30.2.5.3).
§30.4.1.2/3, however, adds:
The mutex types shall be DefaultConstructible and Destructible. If initialization of an object of a mutex type fails, an exception of type system_error shall be thrown. The mutex types shall not be copyable or movable.
§30.4.1.2/3 adds another requirement, specifying precise error codes to be used by mutex types.
Bottom line: a mutex is Lockable, but adds a few more specific restrictions as well.