No it isn't. Battle Mode was in the original game. Two of the modes in MK8DX were also in MK8 and are unchanged. All they did was add the arenas they should have had in the first place, and add 3 new modes and two items. The 3 new modes aren't even mechanical reworks either - one of them is just "Balloon Battle with only Bob-Ombs", the other is "Coin Battle except there is only one coin at a time and you chase the person who has it". These two modes were made using tweaks to existing code that could be done in no time at all. The only extra work they had to do was re-program AI (which doesn't take too long either because they had an AI system in place, they just had to re-program it a bit for the new arenas), and add the UI, and of course general bug testing to be safe. Renegade Roundup may have had more original programming to it, but part of the function here is also having a permanent Piranha Plant which was an item in the original game.
The items are hardly anything new either. For one, the Super Leaf already had a model in the original MK8:
As for it's function - it's literally an on-demand trick ramp, with the same "stealing" property Super Mushrooms have in Battle Mode. The code was there already.
Boo had a model in the game already too since they were stage hazards in one of the courses. This applies to King Boo's model as well, they just made a crown for him. The Boo item may have been programmed from scratch though in terms of what it actually does.
Dry Bones also had a model in the original game as he was also a stage hazard, and they probably just used Koopa Troopa's animation skeleton (...lol) for his playable appearance in MK8DX.
The Inklings are clearly just pulled from Splatoon itself, they're clearly the same model and the two games are made by the same team IIRC, so it makes sense.
So yeah, I'm not seeing a mechanical overhaul. Almost everything was made using existing assets or mechanics.
Maybe it is, maybe it isn't, depends on perspective. Still could have objectively been a better deal for all though, that would most likely mean more profits for Nintendo too.