I said all that to say that despite being loved and a part of the original 12 she doesn't really appear in any of the promotional material for Smash or in a game play mode like Subspace.
In Pokemon despite other popular gen 1 pokes getting stuff like Z moves and megas her line continually does not.
Do they really care about jiggs if she is not shown in anything (but the anime) or given anything?
I don't know.
To be fair, I do understand the point you're making. Jigglypuff isn't a promoted Pokémon at all, especially compared to others.
Whilst Pikachu is obvious, Charizard and Mewtwo both got two mega forms, Lucario was heavily used to promote the mega form, all of these guys also appeared in Pokken and even Greninja was given a unique one of a kind form.
Meanwhile Puff has...Pokémon conquest a bit, the anime(which she's been absent from for years and years) and when they decided to give some Kanto Pokémon new looks for Alola Jigglypuff wasn't amongst the chosen.
Vulpix was pretty much the face of Alola forms with Meowth, Exeggutor, Raichu, Geodude, etc following suit.
Other Kanto Pokémon have received alternate evolutions, forms and mega evolutions. Puff got a baby form that was outshined by both Togepi, Pichu and arguably Elekid too.
It's clear Jigglypuff has never been a Pokémon gamefreak have massively tried pushing so her inclusion sticks out a bit comparatively.
Her claims to fame are being catchable in every region(although I'm sure she isn't the only one) and appearing in this game.
That's it. She's not got loads of merchandise, she was absent from the anime for years and she really is the oddball amongst the seeming uncuttables.
I say this as someone who doesn't wish for her to get cut, as she's the only Pokémon I enjoy playing as, but I can see why she's the least notable Pokémon in Smash history to make it as playable bar arguably Ivysaur.