what do we got here :
new bridge , i think it looks more like portal 2's bridge ? i don't know
new laser , but i think it's messed up
new coins , mushrooms and enemies .
not shown in image :
new bowser
changed the smb tilesset
better portal tilesset
So is this a mod where you change the game's code, or a texture pack where you change the game's images?
Yes
You could've easily done this by taking the game's graphics and replacing them with your own, as Qcode mentioned.
as i said it's not something impressive .
EDIT 2 : here are some new textures