if you deflect, or counter ultimate with a spell, sure,
but fading out of Grav, is the likes of: “being hit succesfully by an ultimate → avoiding consequences of being hit by an ultimate”
that is never ok, so no, it’s not fine.
First of all, if “Ultimate” wasn’t “Ultimate” the idea of calling it an “Ultimate” would be meaningless,
second of all, Moira doesn’t suck, her kit allows her sustain with her self heal, consistent DPS with beam that barely requires aim just general pointing at enemy, her AOE heal is quite strong, stronger with her healing orb, and her ultimate is very good at AOE damage pressure and AOE heal at the same time, not by small numbers either, her ult has some kind of kill/resource pressure on the enemy.
she does get her 100 armor but no longer gets any overhealth/overarmor like in ow1, only her team will constantly get overhealth through anti while brig herself just gets the 100 instant armor hp pool and contant self regen uptime which the regen is stopped by anti