The shadows look a little too dark on the Wooly World stage. It looks phenomenal, though! Great job!

Smash U Link's face does look pretty strange, at least from the front.

http://www.mariowiki.com/Robo_Mario
Another good example of a Mecha/Robo Mario.

Judging by this reference, I think your Mario's cheeks are puffing out a bit too much. The transition from the rest of the head to the cheeks could also be smoother.

Huh. That's interesting. I guess using it as reference would have to be enough. Also, the Falcon does still look a little bright, specifically the blue suit.

I'm rather inexperienced with this whole scene but I'm willing to take a crack at editing and modifying some textures to get them more on-model.
Another thing to consider is ripping and using/referencing the textures from the 3DS version of the game, since those have more accurate colors and shading.

I'm sure with some editing (or complete redoing) they could be made to look good.

specially the characters that had a considerably better design in wiiU!!

That's why I'm especially hyped for actual Wii U Sonic.

Is it just me or does Mac's skin look too light?

Being cross-eyed can be typical with more cartoony characters but it's evident you're going for realism there (even though that doesn't fit Smash Bros 64 very well).
However, the thin part might actually be on the fault of the model itself. I dunno.

Samus' face looks a little thin and cross-eyed.

The eyebrows though...I'm assuming this is the 3ds model, so did you edit the eyebrows or are they actually like that in the game? They just seem a bit taller than normal.

He has two sets of eyebrows for different expressions. At least that's what I assume from the gmod version of the model.

I'm new to this whole scene, but if I could get the 3DS model(s) for Sonic that'd be great.