>>56860676As controversial as it was, she also made more sense. Mew is typically seen as a female Pokemon, both in terms of merchandise and explicitly stated in the original RGBY, where she literally gave birth to her clone; it makes sense that a clone of Mew would also be female, like her genetic donor - especially if the cloning process used to make her was via Genetic Nucleic Transfer the way it was heavily implied in the first games (it's the only method that was known to work back in the 90's when the games were made, and the only method through which the donor is impregnated with their own clone the way the journals imply. This method of cloning only works on females, and only produces female clones).
Giving her the ability to turn into MM-Y made sense, too - it's basically her embracing her nature as a Mew, which works considering she's a far less hostile Mewtwo than the male one we saw in the first movie. Mega Mewtwo Y is Mewtwo becoming more like Mew, while X is more like Mewtwo going off and forging their own path.