Getting jacked is a physical process that requires mechanical damage and healing to individual muscle fibres all over the body in every muscle (unless targeting specific groups only).
Placebo effect is possibly the result of people actively influencing their hormone levels and balance to encourage hormones that promote healing. Specifically increasing serotonin, that we know plays a big role in sleeping, which is where you do most of your healing. Serotonin also makes your entire food to energy pipeline function. More energy allows more energy to be dedicated to healing. Serotonin is also used to create ATP, which activates the muscle fibres.
So I would guess that you could probably increase the speed of getting jacked via placebo effect because your chemical balance is altered to promote serotonin. But you couldn't be sedentary and get jacked. And you could maybe not outpace someone else getting jacked if they're spending more time exercizing than you.
Disclaimer: I am not medically trained, I'm just someone who has been forced by circumstance to learn about the endocrine system and its tyrannical rule over organisms.