By many in the interwar era, it was regarded as such.
The maps in Frank Capra's WWII propaganda film, "The Battle of Russia" showed no border between Outer Mongolia and the Soviet Union.
What short-term, medium-term and long-term differences would it have made if Outer Mongolia were made a Union Republic of the USSR in the 1920s or 1930s?
Would there have been any short term cost on the USSR that was avoided by not having Mongolia in the Union?
Is it enough to decisively change any policies of the Chinese Nationalists or the Japanese?
Could it somehow impair the ability of the Chinese Communist Party to come to power?
If there is a Communist China, could it make a Sino-Soviet split worse?
Presuming the USSR ends as it did, would it change post-Cold War PRC policy in Inner Asia in any significant way?