I think that all you can say about the modern Ainu is that they have more Caucasian characteristics than non-Ainu Japanese who are closer to the Koreans in genetic ancestry.
A more intriguing thought is the steppe mummies found in western China who have no visible oriental features and blonde hair. It brings to question what are the races except for semi-convenient terms of recent ancestry. Since there is no fertility problems, using the word 'race' for such is really a mis-use of the word except that there is no other working term for the results of the 'expression' of dominant genetic characteristics.
The irony for such racial purists as the Nazis and such is that the Arayan[sp?] profile of blue eyes and blonde hair is recessive in almost any hybridization. In other words, it appears that they are the weaker genes.
Tha Ainu are NOT Caucasian.