The story is based on the conceit that a famous actress (Natalie Portman) is set to play the Mary Kay (Julianne Moore) character and goes to her home to interview / shadow her. It's bizarre to think she'd be allowed in, so I think the actress represents the public's desire to get inside the May December relationship. At any rate, the plot doesn't hold together.
As many have commented, Portman and Moore are fine actresses. Moore portrays a delusional, fragile, and manipulative Mary Kay, oblivious to the wrongness of her actions and unwilling to allow her husband any voice. He (Charles Melton) comes across as someone in arrested development; inexperienced and confused.
Portman's actress character is seductive (psychologically and physically) in getting people to talk about the couple. Initially presenting as sweet and respectful, it gradually becomes clear that she is self-absorbed and lives in her own universe. With increasing confidence, she develops a version of Mary Kay very different from Moore's character. It emphasizes the lurid sexual nature which, while true, is reductive and judgmental of the characters and their family members. She particularly targets the child-husband. Portman's character is a far cry from her prior roles, and she delivers a tour-de-force performance.
So the actress / device ends up reflecting back what most of us already think. And it is clear that the characters are much more complicated. I think that's the point. But the roles aren't fleshed out enough and the film is ham-handed in its depiction of characters. For example, one of Mary Kay's sons from her prior marriage is an over the top trainwreck.
The musical score is overly loud and horns in at odd intervals (perhaps reflecting a particular scene in the movie ... but I digress).
Ultimately, I agree with the real Vili Fualaau. who had no part in the making of May December. He said, “If they had reached out to me, we could have worked together on a masterpiece. Instead, they chose to do a ripoff of my original story."