I think it's more of a joke.
Most of the time I've seen it used, it's been used by people referring to -themselves- as furry trash rather than it being an insult aimed at someone else. (Like, "Ehrmagerd, I just paid $9001 for a fursuit by a maker I really admire. I could've gotten a car or something for that money. Once "Furry trash" is added to the dictionary, there should be a picture of me put next to it as the definition", or something similar.)
Tho it wouldn't surprise me if it was being used as an insult as well.