It is thread-safe as far as single read or write operations are concerned. Each get or set action is executed as a single transaction on the database.
When using multiple read or write operations in different threads, race conditions can thus occur as the database transaction always only covers single read or write commands.
The actual threading mode depends on how SQLite is included by Qt Quick Local Storage, but as the default threading mode is Serialized, I assume you can also expect the Qt Quick Local Storage to fully work in multi-threaded environment. To be 100% sure about the SQLite configuration and for more information on the actual implementation, please consider asking in the official Qt forums.