I can count the number of people who haven't used Marx as a blatant counter-pick to Dee with only one hand.
Same thing applies to other characters who apparently are "better picks" for Smash like the rest of the Dream Friends; even though from what I've seen it really just boils down to "they look cooler" or "moveset potential" while completely ignoring whether or not they would genuinely mean something for the franchise from their roles and appearances in comparison to other characters.
For example, would Kirby, Dedede, Meta Knight, and Daroach as the Kirby reps sound "right" to you?
Granted, there's nothing wrong with wanting any Kirby character in Smash, but for everything that he's worked up to, and is continuing to work up to, it's pretty clear that we want Bandana Dee in Smash for more than just his appearance and "moveset potential". Bandana Dee is who HAL chose to team up with the Main 3 in 2011 while remaining as one of Kirby's best, mainstay friends since then. Bandana Dee is whom HAL definitively entrusted the Spear Ability to after time went by, a weapon that hasn't been clearly worked with in Smash yet (with Parasol as well). Bandana Dee was chosen as the true, hard-working representative for an iconic species that has been with the Kirby franchise since the very beginning, This is why I'm glad that Smash doesn't always go for characters who look "cool" and have the most "moveset potential" because it would be a disservice to certain characters who really worked their way up within their respective franchises in one way or another. So, while any video game character can join Smash, that doesn't necessarily mean that certain characters will just because of a few opinionated arguments.
This is why Bandana Dee still has a chance, not only because of his popularity, but because of what he practically stands for:
Growth, determination, and friendship... (and apple juice).