Yup, I've got two questions about this game and I'd appreciate it if someone who has played this could enlighten me.
 
How much do they reuse art assets for the planets and galaxies from SMG? The first game had like the bee themed galaxies, the beach/hawaii style ones, I guess ice/lava ones, those Bowser jr. airship stages and so on. So yeah, are there lots of new themes for galaxies and planets?
 
Also, how much of the levels are in that side-scrolling perspective? I'm asking because personally I want my 3d mario games to be from behind-yer-back 3rd person perspective so yeah I'm hoping those are real minority.

I have only seen worlds 1 and 2, so my answers may change when I've seen the entirety of the game:
 
1. Not much. Every galaxy is themed differently from the other. Though there are of course reused assets. I have only seen 1 planet that was very similar to one in SMG1, reused with a twist.
2. A quarter maybe?
 

1. They might reuse a fair amount of assets, but it really hasn't come to my attention all that often.  Really the game is more about the shapes of the lands, but you will see familiar enemies, those bee hive honey walls, those crystals you spin to break...
 
2. From what I've seen so far (world 5 now), I'd say it's about 1/8th side scrolling type.

I'm in World 5 right now and I did play a bee themed level which used the same song from the first game, but slightly different sounding, a beach/water level, and a ice/lava level.  They are completely new levels though, just with similar themes.
 
As for the side-scrolling parts, I've done some levels with it used.  So far it's been mainly the 3D stuff.

1. Many of the same themes from the first. But every time I go into a world worried about it being to similar to something in the first one, it smacks me over the head with it's great new level designs.

2.  Not that much 2d, and when it is, it usually blends it together with 3d in some way. 

The themes are pretty similar for most of the galaxies, but about 90% of the game is pretty much new, might even be more. 
 
As for the side-scrolling, I actually feel like there's more 2D and over-the-top platforming this time around, but the overwhelming majority of it is from the standard 3D perspective.

Thanks for all the answers.