Moon Girl is not in the MCU, she lives in a universe that is similar in the large scale things but divergent in a lot of others. As Devil Girl put it, "We live in a multiverse!" Even if Moon Girl shows up in a live action movie, it won't be exactly the version from the cartoon. If you want a simple divergence point, this is a reality where the Beyonder took an interest in Lunella, and so manipulated events to be more favorable to her development. Alternately, Lunella herself led to some of the changes, butterfly-effect style (e.g. having to keep an eye on things heating up in the LES kept Maria Hill away from the events surrounding her MCU death). The series kicked off with pulling Devil Dinosaur out of another dimension, so there's going to be divergences.
---Dave