As much as I am arguing in favour of her indeed using her forcefields/bubble barriers to protect Mario and Peach, I feel I should point out we actually don't know with 100% certainty. It's just the most likely method she used as shown by my above arguments.
As for focusing on who she protected and who she didn't... "them" is a very vague term and subject specific. It's only due to Mario and Peach being the subject that we know this. Which, of course, means the focus is pretty limited. Which brings me on to my next point which is... going to have to be a long one.
I find the idea of Rosalina
only shielding Mario and Peach to be very obscure. What reason would she have to protect them? Well, Mario has been collecting Power Stars, right? You know who else has been collecting Power Stars? Captain Toad, as shown in
two missions. And that's only the ones they succeeded at, take how the
Toad Bridage attempted to fight Kamella, clearly due to
the Power Star she flaunts. They were cut short by
being crystallised, but
still show eagerness to complete the mission as Mario would. Bank Toad even
erects a canno , which allows Mario to
reach a planet he otherwise wouldn't be able to. They'll even
pilot the Starshroom in such a way to
grant Mario a shortcut. Whilst not in this game, Captain Toad has even shown to overcome his
famed inability to jump just to track down a Power Star, a loyalty he should still have in Galaxy 1. And the one that speaks to me most is how the Toad Bridage actually
risks their life and limb in Bubble Breeze Galaxy! And this doesn't go unnoticed, as
Rosalina does recognise them to be Mario's friends, and is shown to listen to them. No doubt Captain Toad bragging about his efforts to help collect the stars reached her ear.
It doesn't stop there, either, as we also see
Luigi announcing he will write Mario letters if he finds a Power Star on his search (
which he does) and he
delivers numerous times. In fact, for his Green Star,
Luigi makes it clear he wants Rosalina to know he collected it himself, and
his quest does seem to be acknowledged. Rosalina even
recognises Luigi as Mario's brother, and so she knows the two are close. (I don't think him
possibly being a space Luigi negates the point too much, as they see each other as brothers, and everyone else does too).
And this is all without mentioning the
other people who helped, by, at the very least, ensuring the stars don't fall into Bowser's claws. There's also
Gearmo with his annoying Bob-Omb missions, who appears to be close to Rosalina considering
he lives in the Engine Room of her ship. I'd like to quickly touch on the most potent example of this, which is in Sea Slide Galaxy, where
everyone comes together to search for Silver Stars, including
Bees and Penguins. In fact, one penguin was clearly eager to help their cause, considering
he exhuasted himself. This group effort didn't go unnoticed, either, as it was such a big event
even Penguru noticed. And so it's pretty possible Rosalina also noticed, or at least was made aware afterwards. And a reminder, they aren't helping Mario in this, they're helping Rosalina;
she's the one who requested the Power Stars to be collected, so this is all for her.
So, yes, it doesn't say Rosalina shielded herself... Just like it how doesn't say she shielded the Toad brigade, Luigi, Gearmo,
those at close range to her, and the many other NPCs who helped out when
the entire cosmos is in danger. Oh, but it did state she helped Peach, who didn't do a thing in the mission to save the Power Stars. Please, do go on about how Rosalina is an unjust, ungrateful, jerkish deity who suffers bias and picks out people based on pure whim despite nothing else ever pointing to that character trait before or after this scene.
Which is a humourous way of basically saying that there were way more deserving people to be shielded that Princess Peach Toadstool. A lot of people risked their lives for her, so it feels really out of character for her to pick and choose like that... I guess one argument you could make in defence of this is that
she acknowledges Peach is Mario's special one, and we see from Super Mario Galaxy 2's ending that
the desire to be reunited with someone important to you is a pretty important force in the universe (and beyond if applied to the franchise as a whole), and so she'd be keeping the two safe in light of this, but even then, that falls short. In the ending of SMG2 itself, she doesn't actually bring up this force due to the bond between Mario and Peach, but instead because of the bond between herself and Young Master Luma. So she should be well aware that it applies to other people, and so perhaps the people she explicitly refers to as Mario's friends and Mario's brother might be worth saving too. After all, we see Mario's desire to save Luigi as a baby, and later saving him in games such as SM64DS, SM3DL and even Super Mario Galaxy itself does indeed fall in line with this force, and it works both ways (Mario is Missing, Luigi's Mansion Trilogy, etc). So, even this reasoning would be awkward, not to mention a stretch because the force wasn't even introduced at this point, no matter how consistant it has been.
So, tl;dr, not only should Rosalina protect more than just Mario and Peach but she
would protect more than just them if she were to stay in-character, or at the very least show gratitude. And as such, if they don't mention her shielding them, it has no reason to mention her shielding herself. Therefore, odds are she was indeed shielding herself. An interesting point is that it's written as though she shielded Mario and Peach solely from the blast, and even you say she was accounting for "who needed to be shielded". If we take it super-literally, therefore, the reason she isn't shielding anyone other than those two is that they're the only people close enough for it to be an issue. If she had no need to shield Luigi and the Toads, people equal to and lesser than Mario (take, for example, how Mario could defeat Kamella, but the majority of the Bridage was defeated), then clearly at their distance there is no threat to them at all from this blast, basically meaning she can't even claim the feat for her own self. Though, as I said, that's only if we take it super-literally. Another point is that Rosalina will make sure that Mario won't fall off her ship. If she's that protective, why wouldn't she help the others in a universal crisis? Once again, not protecting everyone seems very out of character for her.
So, yeah, I think the idea of it not mentioning people being shielded when they clearly should be is rather silly, and taking into account Rosalina's character, we can conclude she most likely did indeed shield more than just those stated.
EDIT: I forgot to mention throughout all of this that the reason she didn't shield the lumas is because they were trying to stun/nullify the black hole...
