At the time that ARMS came out, you had Zelda, Mario Kart, and indies. Here are the games of note that had released since Switch's launch: 1-2 Switch, Breath of the Wild, Mario Kart 8 Deluxe, Super Bomberman R, Lego City Undercover, Disgaea 5 Complete, and Cars 3... And there weren't even that many Indies available by June 2017 either with Blaster Master Zero, The Binding of Isaac, and Shovel Knight being the notable ones. There just wasn't much of anything to play on Switch when ARMS came out. Most people had bought a Switch and Zelda, so Zelda was mostly beaten by people at that point. Mario Kart 8 Deluxe was great, but it wasn't really new. And the next big release was Splatoon 2 which was still over a month out. It was the perfect window for something like ARMS to release since there was so little else to play on the system. And anecdotally, I saw a TON of people drop the game shortly after release and also several people admit that they stuck with it longer than they should have. I remember very vividly that time and how excited people were for any Switch release because there was so little available. And ARMS was the first genuinely new experience on the console since 1-2 Switch from Nintendo proper.
I'm not saying it didn't have any dedicated community or genuine support as a game, but I do think its success was largely inflated due to the circumstances of its release and it would not perform as well if it was released in today's Switch climate as it did then.