The use of the 'Garfield' character for the purposes of this parody qualifies as fair use under the Copyright Act of 1976, 17 U.S.C. sec. 107. See Campbell v. Acuff-Rose Music (92-1292), 510 U.S. 569 Comic by XKCD under CC-BY -NC at http://xkcd.com/78/
Article 1, Section 8, Clause 8
To promote the Progress of Science and useful Arts, by securing for limited Times to Authors and Inventors the exclusive Right to
their respectiveWritings and Discoveries;