I certainly believe it was never worn by Queen Mary, and I have never seen a pic of her wearing it.
There are references in Pope-Hennessy:
" 'Alix retaining the lovely little Crown I do not approve of', wrote the Grand Duchess." referring to correspondence between Queen Mary and her Aunt Augusta.
About her first State Opening of Parliament, Queen Mary wrote "What I liked best was wearing Grandmama's crown..." which is believed refers to what we now call Queen Victoria's Regal Circlet. Photos taken in the robing room at Parliament House show Queen Mary wearing that circlet.
The word crown was used frequently for any full circle head ornament, and even by some jewellery manufacturers today.
Edit added: The Crown Jewels 1998 states "Queen Victoria, appreciating its importance, left the piece to the Crown." Also "Queen Alexandra relinquished it reluctantly to Queen Mary some months after the death of Edward VII in 1910 and continued to look upon it with a proprietorial eye." though I feel this is now an error. Also "In 1937, King George VI added the Crown to the Regalia in the Tower, though it has never been used at a coronation ceremony."