When it comes to Party and Tennis on Wii U, it's quite simple as to why they're underwhelming and why Tennis lacks so much content. Nintendo have small subsidiaries turning out HD games in a short span of time. Both Camelot and Nd Cube have 50 or fewer staff, and Nd Cube worked on both Mario Party and Amiibo Festival this year. From what I've heard about Tennis what is there plays brilliantly, and zips along at a rock solid 60fps, but obviously Camelot haven't had time to make new stadiums, more characters, animation cycles and modes, all of which require more work than they did in previous generations.
The manpower requirements for development are much higher than they used to be, and although Nintendo have expanded in recent years, it's been obvious they haven't kept pace with the demands of modern development. They admitted as much when they said they were surprised by the difficulty of their transition to HD development. They've turned out brilliant games, obviously, but what we've seen this year with underwhelming releases on Wii U are the cost of Nintendo's mistakes at managing their internal resources.
3DS struggled after launch and needed more software support thrown behind it, which had a knock-on effect on Wii U development, which was already struggling because Nintendo under-estimated the demands of HD development; Wii U's commercial performance has consistently been terrible, which means third party support evaporated, which meant there was simultaneously more pressure on an over-stretched Nintendo to supply more software to a system which would never guarantee strong returns. 2015 is simply the year when Nintendo's mistakes have most obviously shown in their software output.
As for the dissatisfaction with the Mario spin-offs, I'd point out that Kart 7 and 8 are arguably the best Kart games yet. I definitely wouldn't say Paper Mario has become 'garbage', but the last two entries haven't been as good as the first two, which were first-class RPGs. Underwhelming, decent, sure, but not garbage. Mario & Luigi is simply a series that hasn't evolved massively, sticking to a formula it nailed at the first attempt--which means critical and fan reaction is always going to fall short of exemplary. As I said with the Sports and Party games (were that many of the Party games ever particularly strong?), Nintendo have been asking their subsidiaries to turn out games that require more resources in relatively short spans of time with the same staff levels they've had for years. And, finally, when it comes to the other sports spin-offs, Nintendo did say that after the GameCube, there'd be fewer Mario spin-offs. That comes from memory, but I will try to find a source.
Finally, I'd argue in Luigi's Mansion 2 and Captain Toad, this generation has two of the finest Mario spin-offs released for sometime, to go alongside two of the strongest Mario Kart games yet.
