I'm not having any issues with Splatoon 2 or Mario Kart 8 Deluxe so far. They're very smooth most of the time... but maybe that's because I got used to play Mario Strikers, MKWii and PlayStation All Stars Battle Royale back in the day, so it's a great step up for me.
Yeah, Splatoon and MK really are "smooth" online, but that doesn't mean they're not laggy. They give you a feeling of playing always in real-time, at the expense of accuracy, hence why it happens so often that you hit someone with enough hits to get a splat, yet they just move away, or stuff like that. In fact Splatoon 2's online is even worse than the original.
Smash isn't smooth, because as a fighting game (and potentially 1v1), it'd be incredibly jarring to see your moves connect but not do damage/knockback and stuff like that, so if the game lags, it forces you to stop for a while.
Anyway this has been explained more in-depth earlier in this thread.