It does certainly depend. I'm not sure how Mario kart 8 does things, (like if it even has spectating options), but it also has a replay type function.Just because Smash has it doesn't mean that Splatoon's engine can easily implement it, unfortunately. That being said, I'd love this feature too.
It will always be technically possible to add spectator mode, but if Splatoon's engine doesn't already have enough features and code in place to make a spectator mode feasible, it will be too time consuming and expensive for Nintendo to consider it. It's not a simple thing to do.It does certainly depend. I'm not sure how Mario kart 8 does things, (like if it even has spectating options), but it also has a replay type function.
I think the engine could always be adjusted to record gameplay. Nintendo has done it twice now with two different genres so they should be used to how to do it.
I would certainly love to see it implemented. Don't know how much use I'll get out of spectating, but there are always crazy things happening in this game, so I would love a replay feature.