Not all. Pookie is just the end product. How one achieves pookie can vary from idol to idol.
For example, it seems Rei achieves her pookie state by inflating her cheeks by sucking air in
Whereas other idols might achieve their pookie state by puffing air out with whatever available air supply is left in their lungs.
Of course the easiest way is just using your fingers to poke at it but this doesn't generate the same quality of pookiness and it ties up one of your hand from doing other cute stuff to compliment your cheeks like what Rei is doing. Notice she's much pookier when she rest her chin on her hand vs when she pokes at it