It depends on the server load. I actually found today's matches to be surprisingly smooth, although some untimely lag did strike once or twice.
So that means Mario's Megavitamins will [tornado fang] it up?
If they're changing trajectory, damage, knockback, or any other such attack attributes, then yeah, I'd imagine.
Basically, aesthetic hacks are fine, they deal with data which is not shared between consoles. But anything gameplay-relevant is going to make it impossible for your game to sync with the others, thus locking up and causing a disconnect (at least, this is what happens when attempting custom stages with players who aren't prepped for it).
Also on DeviantArt, Rumble, DLive.tv, and the Fediverse (@freespeechextremist.com and @bae.st)