Aint the simple answer one is a console game the other a cross platform game ?

All shots Nintendo has released thus far are indicative of native 1080p simply by looking at the aliasing. The aliasing matches pixel-per-pixel with what one would expect of a 1080p shot. (Yes, I loaded several images of Smash IV on my 1080p TV, each image matched pixel-per-pixel, unlike their release shots of MK8 and other 720p games, which obviously don't match up).

If it does turn out to be native 1080p, (which seems to run contrary to Nintendo's habits of late; all their other major Wii U games have been 720p, save for Wind Waker) that would certainly explain the lack of fancy effects compared to Mario Kart; with twice as many pixels on screen, that's less fillrate left over for things like materials.

Yeah, in a lot of the close-up shots of the game, you see a lot of imperfections that sometimes have me wondering if the game even looks that much better than Brawl (then i go back and look at Brawl and realize Brawl just looks worse than i picture in my head).

What we really need now is more talk on what makes the Wii U version unique.

Both look really good to me. Mario kart 8 is really outstanding though!

again you're comparing games that arent even in the same genre. As for Killer Instinct...nope im sure it cant look as good seeing as KI is running on HW more powerful than WIi U by a good margin

It's most likely due to being 1080p/60fps instead of 720p/60fps. That's an awful lot of extra resources (225% more in-fact), so it's only natural the in-game visuals would take a hit. Even the PS4 and X1 have difficulty pulling off high end visuals at that rate, so it's not remotely surprising the WiiU can't.

Still, SSB could look better, Even at 1080p/60fps the WiiU is perfectly capable of more.

What techniques would that be?  MK seems to have very simple gfx.  High poly counts, but simple effects.

Genre has no baring on aesthetics and Mario Kart 8 looks as good as Killer Instinct so it's not the hardware, it's the art design.

Common mistake. It's 225% as much pixels, so 125% more.