Stone Skin seems crappy, but then fight mechanics make it good on many of the harder fights.

For example, on Ifrit HM he frequently teleports out to do his charges, when you can't hit him. Once you're in position - free stoneskin. Similarly before hellfire. As soon as he teleports away - free stoneskin which will absorb a majority of the dmg.

On Garuda, During Phase 1 she teleports to do her mistral song while you have to LOS her. Perfect time to stoneskin, assuming you make it there with time to spare (there might not always be enough time on this one). Her big AOE she casts that breaks the rocks is another time you can use stoneskin on yourself, and it should negate most of the damage there. Later, she teleports away for ~3 seconds every time she spawns adds. This is also an opportunity to stoneskin.

there are honestly less opportunities in most dungeon bosses. Those tend to be more about burning the boss without as much off-time.