You can already do this with some N64 emulators with built in netplay like, Project64KSE. There is a small community dedicated to it with a website here.
Smash Bros Melee is much more popular to play online nowadays, and there is a great update for online play called Project Slippi. It works with the dolphin GameCube emulator and makes it very quick and easy to find games against similar skill level players. It also adds rollback netcode, stats, and other QOL features.
If Nintendo, themselves, put out an online Smash Melee remake, it would never be as close to good as Project Slippi already is.
Netplay isn't exactly ideal, as from what I understand it generally requires the syncing of all players emulated console hardware simultaneously (basically, every emulator tricks the game into thinking they're all being played on one single console), which is a lot harder to reliably achieve than having native netcode to handle multiplayer
Slippi, in particular, has rollback netcode built on top of the emulator. It is way less laggy than Smash Ultimate and most other fighting games on the market, for that matter.
It is used for high level tournament play by players who have played the game in person since its release, with no complaints. It's really impressive how smooth it is.
I think you are right about general netplay. Some emulators are better about it than others, and fighting games are of the type where the lag differences will be especially noticeable.
Well holy shit. Nintendo's frankly terrible online for the last several Smash games has always been particularly offensive to me. As I've grown older all my friends stopped caring about Smash so online play was the only way I'd get to play with anyone. Even with gigabit internet not once did I get a match I would describe as good. The closest to tolerable was at least half a second of input lag. That's for the last 2 games. It made me so mad, like why fucking bother putting online play in your game?