Unfortunately, I don't think Bandana Waddle Dee will get in Smash Ultimate
(a thread including things I know from Sakurai's Famitsu columns translated by SourceGaming and PushDustIn)

Before development, all the base characters were already chosen, and a spot was reserved for the new Pokémon from the new Pokémon game at the time (Sun and Moon). Sadly, Bandana Dee wasn't one of them. We all want a definitive explanation, but that will never happen. Here's why:

If someone were to explain why one character wasn't in, it would become an endless loop of "Well, why isn't that one in, then? Or THAT one? Or THIS one? Why was THIS one chosen instead?"

For DLC pass 1, the development team didn't expect this to end up happening at first. Nintendo chose the characters, but Sakurai still had a say in whether to reject them or not and probably if he wanted to suggest a better character

For DLC pass 2, I think they knew it was coming this time. Maybe not idk. Anyway, it was announced at the end of the Byleth presentation. Nintendo has already chosen all the fighters. Recently, the first fighter was announced which is an ARMS character, but we don't know who yet

The reason I don't believe Bandana Dee will be included in the DLC is because he already has a spirit. I know we're all sick of the "Spirits deconfirm!" argument, but this is what I think:

If they announced a fighter that was also already a spirit, it would look unfair to the other spirits, and we'd go back to the third tweet of this thread

People would go crazy asking for reasons why other spirits weren't chosen to be characters, especially if the ones not chosen end up being characters like Geno and/or Shante. If an ARMS character who's a spirit is announced, well, I was wrong. Maybe they're all in and Rayman too

Lmao we have yet to see. I'll be happy no matter who it is because they're all feel refreshing and fun in their own way in my opinion