Anywhere you go on that level, if you look up and to the left, they'll always be there. You can't come any closer. You never meet those "people," and nobody in the game ever mentions them.
They're just THERE.
So this is basically a video game version of the Slender Man urban legend. Fans have already started seeing them in other levels, writing fan fiction stories about them and speculating on what they could be: Local villagers (that is to say, aliens) watching Mario from afar? Those weird-looking giants from Legend of Zelda: Majora's Mask?
But couldn't they just be, we don't know, trees or something? Well, somebody got curious and started sifting through the actual files of the game. And this is where things get really fucked up.
Turns out the sky pattern for that area is called "BeyondHellValley." "Hell Valley" isn't the name of that level, or any level in Mario Galaxy. It doesn't even sound like something you'd find in a Mario game. As for the shapes themselves? They're called "HellValleySkyTree." Ah, see! They're just trees. Now let's pull out the image file itself ...
THEY ARE NOT TREES.
So what the hell, Nintendo? Some claim "Hell Valley" is the Japanese name for that level (which wouldn't be surprising, actually), or the name of a beta level that was abandoned ... but none of that comes anywhere near explaining why those dudes are there and what we did to make them look at us that way.