If anything, Rosalina could potentially become an overpowered fighter when you take into account her offensive versatility and range. Being tall and light might sound bad when you look back at Melee Mewtwo, but that could end up being irrelevant if the fighters can't even safely approach Rosalina.
If the fighters don't have any efficient long range attacks, they'll pretty much be in trouble right off the bat. But again, because we don't know how strong Rosalina's attacks are, the Luma may have to be used to build up damage before Rosalina can try to send her opponents flying herself.
One thing seems certain though; the Luma could be used to save Rosalina from any grab games that she gets caught in, potentially rendering chain grabs useless against her.
I don't think that Rosalina will be so much a weakling as she will be a slow attacker w/ little mobility.
In the trailer, her jumps look amazing, but what about her air speed? And I honestly doubt Rosalina will be mobile on the ground. From what I see, her style is sit around and have her Luma fight. While Luma looks pretty effective, once Luma is KOed, she is a sitting duck. And while yes she can summon another one, but in the trailer, I think the 2-3 second wand twirl was the animation for summoning Luma. She's bait to being attacked and she has such a large open window (now if she was invincible during the animation, then that would be OP as heck). Furthermore, I wonder if Rosalina STARTS with Luma at the beginning of a match without having to summon one manually. If she doesn't, then I can see characters w/ long range projectiles that induce stun (so no Fox) can prevent her from summoning, such as Falco, Pit, and Mega Man. Rosalina looks like she'll counter a lot of characters in close range, but characters that outspeed her attack wise (Fox), can outmanuever her (Marth, Sonic), or can outcamp her and not worry about their projectiles being reflected back at them too much(Samus, Mega Man, Falco) should be able to handle her solidly.