Rosalina hasn't lost her mysterious edge because we still haven't learned anything substantial about her character since Galaxy 2. In order for a character to stop being mysterious, the answers to their big mysteries have to be revealed. That has not happened yet for Rosalina.
To compare to a character who HAS lost all mystery - look at Lucina. In Fire Emblem Awakening, Lucina started out masquerading as Marth, fighting alongside Chrom on occasion. Then it's revealed that she is actually his daughter from the future, and is a princess in that time. However, her future is a doomed one that she intends to save with Chrom's help. Once you know these things, there really isn't any major mystery left to Lucina's character. You can even marry her and get extra close to her.
Not only has the big mystery been solved, but basically every cameo appearance Lucina has made since Awakening spoils this big mystery about her character, even official bios posted by Nintendo online.
Rosalina by comparison, doesn't get her backstory spoiled in bios or cameo appearances. All they mention is that she helped a Luma search for it's mother when she was little, but nothing else, and we know a fair bit more is shown in the storybook. But even still, Rosalina has plenty of unanswered mysteries regarding her. Like where did she get her powers? How much of her power is connected to her wand, and how much of it is innate? Why has she stopped ageing beyond a certain point, allowing her to live seemingly forever? Who were her parents and what was Rosalina's relationship with them like? Who was her brother and what was he like? What exactly happened to Rosalina's family to leave Rosalina all alone? How did Rosalina forget about what happened to her family and live in denial for so many years, thinking they were still alive in the vast cosmos? When Rosalina's mother appeared to her in a dream, was it just a simple dream in Rosalina's own head, or was there a force at work that allowed Rosalina's mother to appear to her in spirit and help her not to lose hope? How the heck can Rosalina have an entire universe up her skirt!?
All of these questions, and more, have not had an official answer presented to us, either in a game or in official material. They were all directly teased in a game Rosalina appeared in though, so it's not fans just over thinking it. They're real mysteries pertaining to the character that have no official answer. Rosalina is a character with many stories yet to be told about her, her life, her role in the universe, and the people connected to her (I mean just look at the amount of books in her Library!).
So no, Rosalina hasn't lost any of her mystery. Being mysterious is not a personality trait, and it doesn't require a character to act a certain way. It's simply a circumstance. There are plenty of characters that are mysterious, but act really upbeat and happy all of the time. That's even usually because they have no interest in sharing the information and want to keep things a secret, so they try to act normal so as not to raise suspicion.
In Rosalina's case, there isn't an ulterior motive to her acting more lively in spin-off games. It's just a more fun side of her that she can bring out when she can cut loose. If anything, I think that adds a bit more depth to her character, as it means she isn't one-note. Plus, this side of her wasn't completely absent from her in the Galaxy games.
As far as her appearing in Odyssey, I'm not sure about that. If they can come up with a compelling reason to include her though, then by all means.
