Coming from someone who has played a lot of Ness, YYG is a novelty.
Even if you could do it reliably, it would only move Ness up a few spaces in the tier list. It's not going to stop him from having a slow, predictable, easily gimped recovery or allow him to ledgeguard or control neutral any better than he does already.
That being said, literally every character is viable in teams assuming you have someone to compensate for their weaknesses or accentuate their strengths. The big problem is that since every character is viable, people will stick with the characters they are comfortable with, and very, very few people can play Ness comfortably at a high level.
While I agree with everything you said, as I play Ness aswell, YYG is NOT a novelty.
(Everything below will be about singles, NOT doubles)
Honestly, in my opinion, if you pull off YYG in one stock, you basically have the whole game. Misconceptions like jacket running out on first hit aren't true. You can u-smash the jacket and make a whole new YYG.
How?
If you pull a YYG, do NOT hesitate to pull off a jacket immediately. In my opinion DJC D-air is fastest.
Then you could either go use another jacket or keep the one you have.
Don't just stand in place either, if you upsmash somebody, the YYG glitch is still active, AND you hit them with the jacket!
Then, another jacket from the STILL ACTIVE yyg!
Even if you DON'T use these tips, if you pull it off you have a guaranteed stock, minimum.
Sorry if I got a bit ranty there, I just am so nerdy for YYG.
Heres some of my vids about it:
https://www.youtube.com/watch?v=fj-DhfjHew8
https://www.youtube.com/watch?v=6daS3GkwDyc