Also called 'parafin,' 'liquid petroleum' or 'paraffin oil,' Mineral oil is a byproduct of the distillation process to produce gasoline. Personally, that's enough information for us to decide it shouldn't go on our skin, but mineral oil has been used in skincare and cosmetics for a loooong time due to its ability to prevent moisture loss. (Ever hear of Vaseline, paw paw ointment or petroleum jelly?)
Mineral oil acts like cling wrap on the skin, locking in moisture. What this means, though, is that it also traps in sweat and dirt, clogging pores and increasing the likelihood of acne and infection. Even the so-called 'cosmetic-grade' mineral oil is listed as comedogenic, meaning it clogs pores.
The cherry on the top of this "no thanks" sundae, though is that it adds nothing to your skin. Mineral oil does not provide moisture itself nor does it contain nutrients that are beneficial for your skin. (In fact, there is evidence that mineral oil is one of the largest contaminants of the human body.) So why use a pore-clogging possible contaminant on your skin when there are so many 100% natural oils, butters and extracts that do actual good for it?