Seems to me the addition of stripes to a basically well-designed car is a statement: "I sense a deficiency about my car's looks, and I'm gong to follow a certain crowd in an attempt at masking it."
A Bullitt Mustang is fully-designed and should be pretty much left alone. Others might be sufficiently remarkable without significant additions or deletions*, and there is no doubt that some need some kind of decoration to take them out of the humdrum range. In many cases changes can only make it worse; in others, almost anything would be an improvement, distracting attention from the run-of-the-millness or plain old ugliness of the underlying design.
Of course there are reasons to stripe up that have little to do with design or pleasantness of impact: I might go with a set of distinctive stripes to declare my loyalty to a certain Le Mans winner...
*Certain colors are likely offered with this in mind; who immediately notices stripe schemes on Gotta Have It Green cars?