Long story short, it suffers from the same 'wouldn't it be cool if.' paradigm, where the idea itself is so self-obsessive that it's anything BUT cool, but in practice, were it even possible, it would make for an insanely deadly opponent. You'd basically only ever really hurt them by dealing massive, unpredictable critical hits, meanwhile they'd be firing away at you with some form of very deadly weapon. If you simply think of it from the perspective of stats and damage reduction, any damage dealt to them would both have to pass through the reductions made by their Power Armor (heavily reducing anything taken to minor irritants) and THEN the reduction of their 'equivalent to very strong Combat Armor' thick skin. But, hypothetically, if a Super Mutant were to be clad in Power Armor made for their size, they would be the not-ridiculous equivalent of the East Coast Behemoths damn near invincible.

That's why there were never any Super Mutants wearing it. For one thing, Power Armor was a thing made in the old world, which is why APA is such a big deal, since it was created in the Post-War world (albeit, from an organization with Old World technology), so Power Armor would never come in Super Mutant sizes.

Fallout 4 hellfire power armor