It isn't, but usually not in the way you'd think: in my experience food truck food beats most restaurant food in the same price range.
I don't find (most) food truck food overpriced at all - the food truck "revolution" seems to be almost exclusively centered around gourmet-on-wheels, rather than street-meat.
If you're slinging around stereotypical burgers and fries you will get no business - the successful food trucks around here seem to have struck a balance between eclectic taste and broad appeal, and the quality of the fare is always on par, if not better than, B&M restaurants.
Is it?