I would guess that this is the nature of the beast, I have noticed, if I am being especially spirited with the bike, I better know where a gas station is, I found going to stearns last spring, I was averaging about 235miles per tank, but I wasn't pushing it hard. regular riding in the city, if I get 120miles off of a full tank I think that is totally awesome.
I really have no clue what good or bad mileage is on this thing, and I really don't care, I didn't buy it to be fuel efficient, I bought it to have fun, and man it has no let me down yet.