What you get from disassembling a NES ROM (or, really, software for any 8-bit computer) is not much different from what went in originally, except (maybe) there were some comments.
It's very unlikely that the original assembly source exists anymore, or even that it was kept much later than the game's release. It's not until very, very recently that game companies started to think that the original source and assets for a game would have enough value to be worth the cost of archiving. Even games from the PlayStation 2 era routinely have to be rewritten from scratch with new assets when re-released on later consoles.
In my experience, most of the game source code that survived from the early years of personal computing is for games written in BASIC, which would have been distributed as source. There's a few exceptions like for Jordan Mechner's games, but that just goes hand-in-hand with his famously detailed records of his development process.
While I agree that the original source code for the game is probably long gone, I think we can settle for having a disassembled version of it. As you pointed out, the game was most likely in handwritten assembly, so the assembly output probably isn't much different. The most useful part would have been the comments, which it turns out there were some very dedicated people that went through the entire disassembly and annotated the source code like this one by doppleganger http://www.romhacking.net/documents/344/ and this one where movax tried to make the assembly more natural http://www.romhacking.net/documents/635/
There are several really cool things you can find on here about older games including a detailed explaination of the minus one world http://www.romhacking.net/documents/343/ where doppelganger covers exactly why the game does what it does.
It's very unlikely that the original assembly source exists anymore, or even that it was kept much later than the game's release. It's not until very, very recently that game companies started to think that the original source and assets for a game would have enough value to be worth the cost of archiving. Even games from the PlayStation 2 era routinely have to be rewritten from scratch with new assets when re-released on later consoles.
In my experience, most of the game source code that survived from the early years of personal computing is for games written in BASIC, which would have been distributed as source. There's a few exceptions like for Jordan Mechner's games, but that just goes hand-in-hand with his famously detailed records of his development process.