The Mii Costumes are absolutely a key selling point of them(literally too). Making your own Mii is important, but the Customization in other ways are directly how they're presented. All three things are very important. You actually can't really do much with them without the Costumes and customizable Special Moves. This is really what made them stood out among others. Especially with Palutena no longer being able to hold the same niche.
They should absolutely throw the Mii Maker in there, because it's actually a great way to keep them while still allowing for players to fully enjoy the most out of them. It's also cheaper than trying to give 40+ characters new costumes. Especially when it's much more difficult to figure out not just colors, but deal with many characters who already had some trouble getting good ones(Pokemon, some 3rd parties), and then you have to remember that the Mii Costumes in many cases are already done. So it's actually more cost effective to put one mode in there, and reuse all the materials you have, with only having to license the 3rd party ones and new ones as they come up.
They're also just plain popular as a concept(and let's be honest, many are more than fine with using the generic design, since the not all Costumes are just hats. But even then, people just love the concept of playing as a silly character skin to imagine they are like that). And yes, the Mii Maker should return too. It's just the only thing they can cut at best but still keep the Mii Fighters. The other two are not possible to cut in any way without removing a huge key factor to their success as an idea. They are custom characters. They'll still be while losing a small part of their customization. It's also important to note that it's the only Customization part that isn't from Smash itself, while the other two pieces make up a huge part of them. All three are preferred, but 2/3 still does enough to keep them fun as a character. Besides that, some just want to outright use them without having to make a Mii first. So it's actually not the worst thing to have a generic set. It's fast and gets to the real meat in the game(the custom moves and the costumes themselves).
