It's been successful in every case, honestly. Almost all the time there will only be a small bit of people refusing to buy the costume, but enough buy it for it to be successful as is. There's been no shortage of mass DLC sales because people want content and nods to their characters, including obscure ones. For a company, they want their product to sell. Refusing to buy something like a Mii costume only shows that you don't want their content in the end. People can argue all day that it's "if it isn't this content, it's not worth it", and that's fair. But there's a caveat; in order for a company to gauge interest of their product alone, they need some way to get the product known. For obscure characters, a Mii costume gets people heavily interested. Good sales means that they can tell they have a reasonable chance of selling that product. For instance, the Isaac Mii costume is the best way to help gauge whether people really want another Golden Sun or not at this point. There's some fan outliers, but keep in mind many companies don't pay attention to these kind of polls as is, or even petitions. Now, companies do look at actual reviews(that is, when it comes to actually improving their games). Especially since Mii costumes, unlike some content, are actually quality content meant to help you enact the fight you wanted. Is it perfect? No. Really, worst is sometimes the costumes look a little strange due to proportions, but now they're all one specific shape due to them cutting the Mii size differences(which is a good thing, as it helps keep them balanced and more applicable for tourneys as is. Since that was the biggest problem with their balance. Now it's just a matter of custom issues, and that's not that hard to test as is).
So I highly doubt having a fan favorite as a Mii costume will hurt the overall game, sales, or even the fanbase. It'll go good no matter what costume they give as long as they're sure the character has fans. That's really important when it comes to figuring out who's really a fan of their product. Geno's an interesting example of this factor too. He's only gotten more Smash content since his costume as is, which is the Spirit in itself. People really wanted him in. He didn't become playable(as of yet), but it did show there's a lot of love for the character. Though I can't remember if the SNES Classic came out after Smash 4's DLC or not. But now Square-Enix absolutely knows how much people really want products related to Geno. These small things are really important for companies. They can't throw out a new game, for instance, if they aren't sure it'll sell well. So they need other product to do so. When they are unsure their franchise will do well, they can't always take a chance with a major DLC like playable. That is, if that is even possible for them(as Sakurai has approached all companies thus far from what I remember reading. Do correct me if I'm wrong). Sometimes they would rather take a chance with a smaller role to be safe. Geno is a good example of this. With Super Mario RPG never getting a real sequel, being sure people really want the character revived is important. Of course, some characters don't need this as it's already clear due to other game/product releases(Banjo), but that doesn't mean something like a Mii costume would be bad to them. In a case like that, they know the character wasn't asked for when it comes to playable. But they know fans will want product relating to Banjo as is, so adding him in only helps give people the character they want in some way.
I hope I explained that well enough, but companies care a lot more about sales than people think. Sales are just that important. With bad sales of content, they just see the related content as a flop and aren't adamant to continue anything related to that product. Right now, in the case of Banjo, he has Rare Replay going for him. This is selling well. But these are just re-releases and not a new video game-related product. Merchandise doesn't do it alone either(it can help, but it doesn't seem like it's doing anything for the Bear and Bird other than at best making sure people know they exist). You need more than that. It's not simply like continuing into a third season of a show, where merchandise sales can help(as it gives the people behind the show incentive to think it'll continue to be popular). Making a new game requires a lot more than continuing a season, namely because with a season, you already have a goal in mind. With a new game, you need a lot more than just a goal. You need to figure out if it's actually part of a specific set of games(as a sequel), who is going to be outright creating the game(at least with a new season you have a team set up), who it would appeal to when it comes to gameplay(see: Nuts & Bolts as proof it's not very easy to make new games for a franchise), and then you have to be sure the game alone will sell well. Though merchandise specific to that game can help too, of course. It's still important, but a lot of games aren't designed to sell well with merchandise in mind.