I know it's been a while and I'm sorry it took me so long, but I guess better late than never.
Finally I'm able to release Mario Maker 2 backgrounds! This pack contains all backgrounds from both Mario Maker 1 and 2. In Mario Maker 2's case, backgrounds come not only from levels, but also from world maps.
List of things I added:
- Mario Maker 1 backgrounds which were missing previously
- config files for all Mario Maker 1 backgrounds
- all Mario Maker 2 backgrounds (duh)
- config files for said backgrounds (exceptions are SMB3 and SMW BGs, will explain below)
- beta and unused backgrounds (& config files)
- backgrounds with custom animations (& config files)
- fixed backgrounds (& config files) - will explain below what those are
- backgrounds which are unusable by the user, like those from the tutorial levels of Mario Maker's 3DS port, or the one from Story Mode's finale of Mario Maker 2 (& config files)
- backgrounds from Mario Maker and Mushroom Kingdom stages of Super Smash Bros. Ultimate (& config files)
- 20 GIF examples
In total there are 120 BGs in the pack.
Almost as big as my Yoshi's Island BG pack.
Here are some pictures for you:
SMB3 Desert 1 (day):

---
SMW Forest 1 (day):

---
SMB3 Forest 1 (day):

---
SMB3 Underwater 2 (night):

---
SMW Ghost House 1 (night):

---
SMW Clouds 2 (day):

---
SMB Forest 2 (day):

---
SMB3 Underwater 3 (day):

---
SMW Underwater 2 (night):

---
SMW Snow 2 (day):

---
SMB3 Night:

---
SMW Forest 2 (day) - custom animation:

---
Story Mode Credits:

---
Space World Map:

---
Super Smash Bros. for Nintendo 3DS - Mario Maker (SMB1):

---
Super Smash Bros. for Wii U - Mario Maker (SMW):

⠀
Download!
Backgrounds are relatively easy to use, just put a BG image and a corresponing config file together.
Some backgrounds, like SMB Desert, consist of two parts. That means you have to put them both in a level and their corresponing config files, so that they can function properly. This may not make sense right now, but after downloading maybe you will understand what I'm talking about.
About config files, "fixed" backgrounds, and why I didn't add config files to SMB3 and SMW BGs of Mario Maker 2:
Config files contain information about backgrounds: they are used by SMBX to determine scrolling speeds, Z-order priority, and whether or not the background in question is animated.
I added config files to every single background in this pack, except for SMB3 and SMW BGs of MM2. Why I didn't add them for the rest of the BGs you ask? Simple answer is that the process of doing that and testing each and every one of them is tedious for me. Even though some refer to me as a "super computer", I'm afraid I can't live up to that title.
So if you want to use those BGs, you will have to create config files yourself or alter existing ones unfortunately. SMB1 backgrounds contain config files, those are good examples of how this can be achived.
Again I'm truly sorry for this. I don't have the will or motivation to do anything related to SMBX since 2016, this is something I wanted to release only because I promised an update if DLC comes out. If I didn't do it today, as unfinished as it is, I probably wouldn't release it for another year or two.
Moving on, "fixed" backgrounds are backgrounds that fix graphical errors by Nintendo. One instance of this:
There are a few backgrounds which I had to fix. Of course, the pack contains both versions of certain backgrounds, so if you really wish to use the "official" versions, you can.
⠀
If I forgot to add a background, please feel free to tell me. Please enjoy.