I don't think the Min Min hint means "disregard everything I said it's just a tease lol", it's just a sensical business prediction.
Generally with DLC, the further you go from a game's release, the more sales diminish. Except for Star Citizen for some ungodly reason. But you know what doesn't diminish? Licensing fees. Third party characters cost money, and Nintendo currently has to have the license to fourteen characters from twelve series from eight different companies just to maintain the roster. Then you have other factors like the music and creating and distributing figures of the characters and you've got yourself a whole bunch of complications. It's very much a risk versus reward situation, and as the risk remains the same, the rewards go down.
On the flipside, you have characters like Byleth or Min Min, who may not be as popular as the third parties, but cost nothing to use. It's not as rewarding, but it's not as costly, either, and the further from launch you get, the more profitable of a route that is. A character like Joker or Hero may have been financially successful right after the game came out, but a whole two years later, it's a very different environment.
I still think there's plenty of third parties with a good shot, particularly from companies that are easier to license from like Namco and Capcom, but I wouldn't be as sure we're gonna see anything too crazy going forward.