I’m pretty sure smash’s online is p2p based, where Nintendo’s servers only really handle matchmaking and making sure you weren’t hacking or anything at the start...
... in other words it’s either your fault or one of your opponents for the extreme lag, but Nintendo’s servers have little to nothing to do with it. Remember since you’re connecting with up to 3 other people, even if you’re not the source of the lag, any one of them could be, or multiple players could, also keep in mind the game is trying to communicate between 4 game consoles from around the region, with multiple different security settings, firewalls, etc.
Be careful what you wish for, if Nintendo decides to go the splatoon 2 route in smash switch and immediately start booting people at the first sign of an unstable connection by them and then eventually start banning them, and you happen to be the source, you might just find you won’t get to play online at all very often. I’ve already had moments where my connection has acted up just a little bit (usually early mornings) where smash would still be playable, just not ideal, but splatoon 2 boots me almost immediately and bans me for awhile.
My recommendation is when you play smash, put everything else connected to your network on airplane mode/turn off their wireless. If you have an app or program downloading, and you try to play, you’re likely to be a source of extreme lag for yourself and opponents.
If you’re 100% absolutely confident that you aren’t the source of your lag(which.... everyone seems “sure” it’s not them) then chances are it’s one or all of your opponents faults. I’d recommend staying out of ffa matches and go to either for glory 1v1 which decreases your chances of encountering an extreme lagger, or better yet avoiding the random online modes altogether and playing against people on your friends list.
About the only thing I would say Nintendo could actually change would be how it handles matchmaking, like, don’t pair me up at all with someone from south or Central America, pair me up against USA players only, even if it takes slightly longer.