Out of nowhere, a new update landed today for Arms, bringing with it a minor functionality change and a bunch of smaller game fixes.
Update 5.2.0's feature update is that Big Time Arms, a mode previously limited to play as a Party Crash mode, is now a playable option in regular local multiplayer versus battles that can be toggled on and off. What this means is that, by pressing Y, players can now swap out their Arms for giant oversized novelty versions of those same weapons. This mode is not supported in online play, but is available for local regular matches.
The main gameplay tweak in this update fixes an issue where fighters with super armour would not have their Arms disabled by electric attacks during a rush attack.
There are also a number of smaller balance changes to characters, which can be read in full here. It's nice to know that, while the new character and stage updates for Arms are all done, that doesn't necessarily mean Nintendo is done fiddling with the game.