Well, no. Plagiocephaly is caused by growth of the head against a firm flat surface, because the baby had torticollis and couldn't turn his head the other way. Like growing a pumpkin against a wall, or another pumpkin, it grows flat where it contacts something else, right? Remove that firm flat surface (eg, baby learns to roll and sleeps on their belly) and the head just continues to grow from the shape it is in. Growth on the flat spot hadn't been "paused", it was redirected elsewhere, which is why the other side of the head sort of "bulges".
There are plenty of studies that demonstrate that well fitted helmet will do the job - the issue is, if the helmet does not fit (due to an unskilled orthotist).
As I said, the FACE will correct, the head shape, no. But unless you are looking directly down on top, with wet or very short hair, you probably won't see it. In an adult, that's pretty rare. In a child, more common, but still only cosmetic. Parental choice either way.
A few papers:
Vles JS, Colla C, Weber JW, et al. Helmet versus nonhelmet treatment in nonsynostotic positional posterior plagiocephaly. J Craniofac Surg 2000;11:572Y574
Littlefield TR, Beals SP, Manwaring KH, et al. Treatment of craniofacial asymmetry with dynamic orthotic cranioplasty. J Craniofac Surg 1998;9:11Y17
Graham JM Jr, Gomez M, Halberg A, et al. Management of deformational plagiocephaly: repositioning versus orthotic therapy. J Pediatr 2005;146:258Y262
Teichgraeber JF, Ault JK, Baumgartner J, et al. Deformational posterior plagiocephaly: diagnosis and treatment. Cleft Palate
Craniofac J 2002;39:582Y586
Lee R, Teichgraeber J, Baumgarter J, et al. Long-term treatment effectiveness of molding helmet therapy in the correction of posterior deformational plagiocephaly: a five-year follow-up. Cleft Palate
Craniofac J 2008;45:240Y245