Tammy Dickinson, United States Attorney for the Western District of Missouri, announced that a Joplin, Mo., woman pleaded guilty in federal court today to filing a false tax return as part of a scheme that resulted in at least $138,000 in refunds.   Tara F. Garrett, 36, of Joplin, waived her right to a grand jury and pleaded guilty before U.S. Magistrate Judge David P. Rush to a federal information that charges her with making a false claim upon the government.   By pleading…