Here's the thing - as I recall ( and this is going back to when I was reading comics in the 60ies, 70ies and 80ies); Mantis first appeased in the Avengers as Swordsman's sidekick and was originally an Earthling (Half-Vietnamese and German) and was an amazing martial artist. As a child she was part of a Vietnamese sect that worship the Kree and the whole deal of her becoming the Celestial Madonna was after she was kidnapped by Kang the Conqueror.
My point: She didn't start out with known Celestial powers (and also WASN'T an alien) - and if they want to take and 'grow' the character into the 'Celestial Madonna' aspect, they still can.
My point: She didn't start out with known Celestial powers (and also WASN'T an alien) - and if they want to take and 'grow' the character into the 'Celestial Madonna' aspect, they still can.