There is a cap, in the arena, for MP Absorb at 16%. If you look at the first image below, Mythstones are scaled by 70%. This means your 10% Return Mythstone will become 3% MP Absorb.
Your overall damage reduction is ~68.5% and it appears you're using Equivalence Mythic with Druidic HP Regen. Try to have 6 or 7 Flat ED affixes on your character. This means Epic 5000 Shock damage on each gear (Mainhand Weapon, Offhand, ... Pet). This will become 1040 ED on each gear in the arena. That should get your damage up much higher and help you improve your MMR.