It can be, but its also just used to refer any individual who is American. Maybe you can feel offended by being labeled based on your nationality, but Mexicans like to use descriptors like that as nicknames for people. We have guero for white skined, moreno for dark skinned, flaquito(a) for skinny person, gordito(a) for a fat person. Take the last example: gordita when translated literally means little fat girl, but its often used endearingly and without the heavily negative connotation the English translation would bring. Kind of the same deal with gringos.