According to an accumulation of stats from NPD for North America and Australia, with UK Sales also thrown in, Mario + Rabbids Kingdom Battle is already the best-selling physical retail third-party game on the Switch.
For any pedants among you, it's worth pointing out that the game was published by Ubisoft and developed by its teams in Rome and Paris; Nintendo, naturally, consulted on the project. It also seems unfair to other third-party games so far as, well, they don't have Mario in them.
Nevertheless, with the game having been out for less than a month it's a notable landmark. More data and figures are also needed to figure out whether it's overtaken any first-party Nintendo games - according to Nintendo's most recent official figures ARMS and 1-2-Switch will be the most likely to get caught.
It's not particularly surprising that the game is doing well, because there's never been a 'Mario' game quite like it and, well, it's pretty darn good.
Are you one of those that's bought and played Mario + Rabbids Kingdom Battle?