It'd be nice even if it was just a cameo, considering Nintendo butchered Star Fox with the Wii U gamepad in 2016.

I doubt it. It’s probably just a quick cameo to set up the Smash Bros. movie.

Then we need the Mario Galaxy Movie to do huge box office numbers so the Smash Bros movie happens.

More coverage of Fox in the movie.

I will say this: it’ll be interesting to see how they explain Fox showing up in this.

We already know from the Kart and Party games Donkey Kong and Mario are canonically in the same universe, so no extra legwork needed for him (and the rest of the DK Crew) appearing in the first Mario (animated) movie. Fox, Samus, Link, etc. aren’t, so barring some incursion/time travel/universe-jumping, it can’t happen. It’s not as simple as him and Mario going to space at the same time in this.

Well Mario & Luigi is already in an alternate universe, so Fox can just have found a warp pipe too.(or a black hole or whatever)

Such a thing isn't unheard of in Star Fox. The first Star Fox had a black hole that could be used to take shortcuts to other levels via warp rings, who's to say it can't take Fox to another dimension or another universe? Could happen to Samus too, as she's been stuck in a parallel dimension before (See Metroid Prime 2).

Did anyone here get to go to the movie's premiere?

Mario, Luigi, and Peach get flung out their world into a new galaxy where Rosalina is, Bowser/Jr. trigger events that cause galaxies to start collapsing on each other, they go into another new galaxy has Fox in it with the hint that there are many other galaxies with other characters in them (Smash Bros. set up). Fox helps Mario and co. get back to their galaxy and then Mario/Luigi/Princess/Rosalina all beat Bower/Bowser Jr. and stop the galaxies from collapsing. 

Easy peasy. 

The galaxy plot device lends itself to parallel worlds with other characters (Samus, Kirby, Pitt, Olimar, Pikachu) easy enough though even the first Mario movie could have set this up with the idea of underground pipes that lead to different worlds. 

I think it could be more than a just a quick cameo because they got Glen Powell to voice the character, he's one of the bigger up and coming stars in Hollywood, I suspect he has maybe not quite as big of a role as DK in the first one, but not a tiny role either.