This article reviews several costumes that spoof being an illegal "alien." Apparently, Target offered this costume on-line, until consumers expressed outrage.
I think there are two ways of reading this: one is obviously offensive, and supports the idea that undocumented immigrants are strange, dangerous, and scary.
The other really spoofs the idea that there is such a thing as an illegal alien. In this sense, the costume points out that the idea of an illegal alien is just silly--the only aliens are the ones that are dreamed up by conspiracy theorists and UFO chasers.