Why Are There Dimples in the Sides of my Butt?

It is hard to tell without seeing photos of your condition.  Sounds like you need to see a board certified plastic surgeon to get a direct answer. 

Hello dear!!
Thanks for the question and provided information as well.
Based on your description, it appears that you could benefit from a fat transfer procedure, but unfortunately, without pictures, it is very difficult to give you a proper answer. l recommend you to make an appointment with a board certified plastic surgeon to talk about your goals and anything you need to know.
Good luck :)
 

It is very common to have the lateral buttocks depression that you refer to. Weight gain may possibly correct this for you, but how much weight will you have to gain to have this area fill in? And, how much bigger will the rest of your body get in the process?
Building up your gluteus muscle may also help.

Fat grafting aka the Brazilian Butt Lift would also be an option if you cannot get the results you are seeking on your own.