Honestly, I'd prefer it if she seemed to have something central to do with the plot. From the synopsis... it seems she's just going to be a performer... and that's it. It seems to be just an extra-long cameo, which is time that would be better spent on THE STORY which I am fearing more and more will be terribly rushed given the sheer number of characters and locations included. It's overload for a movie that is likely to be at most an hour and a half long.
Think about it in terms of cinematic development: 7 core cast members (Mane 6 and Spike), at least 3 central antagonist roles, at least 6 supporting/secondary cast roles in two major secondary locations both of which require elaboration and THEN fitting them into the core plot after introducing the new theatrical audience to the world of Equestria and the gist of how things work there. And now we have a large cameo role eating up at least 5 minutes (every minute lost to anything other than the story business in a cramped plotline is a waste of time).
Remember, the movie is trying to reach a wide audience. That means telling/showing everyone how Equestria functions in general and why the Mane 6 are important characters we should care about. This is a weird magical world with weird magical rules governing it. At some point, the movie MUST give it a structure or no one outside the fandom will have any idea what's going on or why. This cannot rely on an opening info-dump. That's one of the Signs of Doom for a kids movie, and many movies in general. The 'prologue' of a movie must not try to shove in too much history... otherwise you get "Dune". But here we have a problem: the Mane 6 are already together and important. The movie audience will want to know why. So at the very least, the NMM story has to be compressed and thrown in there to explain why 6 seemingly random ponies are deciding to go save the world from the Big Bad.
Many animated films struggle to carry more than 3 main characters and 2 villain roles with a smattering of lesser cast members when only in one location! This story needs to fill in the audience on a MASSIVE amount of background to set the stage in the first place before the story can even get started.
Look at some classic examples of how NOT to over-burden an animated story:
"Land Before Time": 4 central characters, a few incidental characters, no real villain save a T-rex with no speaking role. One basic quest to find a new home. Straight to the point, plenty of time to develop everything fully and have room for pathos and 'quiet space' for mood and tone.
"Aladdin": 3 central characters (Al, Jasmine, Genie), 2 villains, a couple of side characters. All linked via one plot thread in one central location with a rather basic set-up that's fairly self-explanatory. Villain looking for magic lamp with wishes to rule the world (Bison: "OF COURSE!), the kid gets it instead and now the plot ensues without interruption.
"Spirited Away": basically 1 main character, everything in the slice-of-life fantasy revolves around her quest to return to the human world. The focus is fine-tuned and permits plenty of ancillary character action and scenery.
In other words, most animation studios with experience and wisdom remember the K.I.S.S. principle of storytelling: KEEP IT SIMPLE, STUPID!!
This story is cluttered with 16 characters of core importance to major parts of the plot and at least 3 very radically different locals. It is almost certainly going to have to move at a break-neck pace to fit everything in. Even Marvel would find this all but impossible. I sadly have to conclude it's going to be one of those hectic kiddie movies where too little makes sense and events simply are thrown across the screen without explanation, resulting in Nostalgia Critic going on one of his famous "EXPLAIN, MOVIE!!! EXPLAIN!!" rants after talking about how the movie was written for ADHD sufferers drinking a cocktail of Red Bull and meth.
Now, this is not to say it won't be profitable. The animation cannot possibly have cost all that much to produce with such a simple style and even with the voice talent I'd be shocked if the total price tag went much beyond $30 million.
That means only a modest showing would be needed for profitability.... and let's face it, kids movies tend to do well enough even when they're poor. The terrible AWFUL "Boss Baby" made money despite costing $125 million (HOW THE HECK DID THEY SPEND THAT MUCH?!) and being a story no one was asking for and hardly anyone had ever heard of the source material save toddlers. This would have to be an absolute trainwreck. I'm talking Uwe Boll-level garbage. It simply won't be THAT bad.