I'm guessing everyone saying that Bandanna Dee is irrelevant or just a standard enemy with a hat or a blue toad have never actually played a Kirby game with him in it. Or if they have it definitely wasn't Super Star Ultra.
From an outsiders perspective he may seem like a lazily designed "player 4" for the kirby series based on a basic enemy with no real relevance, but that simply isn't the case.
For starters people need to stop with this comparison of waddle dees to goombas. They are NOT the same. Sure they are both basic enemies from their respective games, but waddle dees carry much more personality as well as individuality compared to goombas. Goombas are pretty much all the same and are just boring henchmen of the King Koopa. Not to mention that in game they are basically moving obstacles for the player to get around. Waddle dees are a different story. Because of the nature of Kirby games they can't really act as any kind of obstacle to the player. So instead they have to contribute to the aesthetic and personality of the level, especially in more recent Kirby games. You can see them driving cars, manning tanks, Sleeping in huts, and even commanding a military. They are may more unique and interesting than goombas on a very basic level in terms of their purpose and design, not to mention that they almost literally can't kill you in game so "basic enemy" my ass. So even if you think that Bandanna Dee is just a waddle dee with a hat and a spear, Waddle dees are still 100x more interesting and unique compared to goombas.
On that note, anyone who has played KSSU will know that Bandanna Dee is the commander of King Dedede's army and that he has a very distinct personality even when compared to characters like Meta Knight. He is loyal and supportive of his King while also wishing for the best of all people including Kirby. He is pure and wholesome and will do whatever it takes to serve his majesty as well as dreamland if need be. People call him the "player 4" of Kirby games when he has really only filled that role twice in any Kirby game (barely so in star allies). In most games he is helping Kirby out and assisting him on his journey to save Dreamland. His main move-set consists of the spear copy ability which is an incredibly fun copy ability to use and his move-set would perfectly translate to smash. He has also used a parasol in Kirby battle royal and has even visibly cracked Popstar by punching the ground with his raw strength alone in megaton punch.
Regardless of whether or not you think Bandanna Dee is a unique enough character to be in smash (he is) Waddle Dees need some kind of representation in the roster, especially since Dedede's side special changed.