I like how people are automatically fanboys just for saying the ARMS cast reminds them of Overwatch's, or that it could be the next Splatoon. I mean, hell, there's both articles and reviews out there that help fuel the comparisons as well... So don't blame the "fanboys" for making obvious comparisons.
Regarding the Splatoon comparison in particular, that's an easy one to make, considering the quirky take on the fighting game genre just like Splatoon was a quirky take on multiplayer shooters, as well as the game following a similar model in regards to free DLC. Why wouldn't someone label it as being the "next Splatoon"? It'll likely be far less popular considering the genre, but the label is really nothing to get so worked up over when there's an obvious basis for the comparisons.
As for the Overwatch one: Colorful and diverse cast. Apparently lots of fanart. Lore around the world, as well as backgrounds for the characters that will be trickled out. Not a hard jump to make there. It'd be one thing if someone said it'd match or surpass the Overwatch cast's popularity, as that would be a silly prediction, but to my knowledge, no one said anything of the sort. Just because people say something is the next this or that, doesn't mean it's this exact 1:1 situation where they're saying that ARMS will literally be just as popular lol