You can check out his ancestry here
http://hailtoyou.wordpress.com/2012/01/20/mitt-romneys-ancestry/
As you can see there is no Mexican ancestry whatsoever
of course he has Mexican ancestry, unless you are saying not having Aztec blood precludes you from being Mexican. Vincente Fox was of German ancestry.
By your logic if I am not a WASP , i a not American. Well Obama is half wasp and no african american heritage .