These last two lines of Canto Four resemble a huge rhetorical "WHY?" or "If only..." uttered plaintively in the wake of the serial mock-disaster caused by the snipping of a lock of hair. After the epic battle between the sylph and fairy armies of Belinda and Lord Petrie, the bloodshed, the family feud, and the "rape" of Belinda's appearance, these lines are cast up to further the satire of Pope's poem.
None of this would have happened, if the piece of hair had not been so noticeable. If Belinda's beauty had not been altered, if her appearance were not in peril, this whole catastrophe could have been avoided. The obvious irony here is that it should have been avoided anyway. A small piece of hair raised to such epic proportions illuminates the vanity and trivial pursuits of the society.