That honestly looks like a mess.
The camera zooming out is a very important feature of Smash Bros, you not only need to know where your opponent is at any moment, but what they're doing, otherwise how can't you properly react to them? Not to mention that half the time you can't even see what you're jumping into on that stage, which is made even worse by the extremely awkward pseudo-isometric perspective they choose (which I guess wouldn't be a problem in Smash if they don't do the same, but it still doesn't help the point.)
Now, they could actually make the screen zoom in and out, the GBA is perfectly capable of that, but with the super low resolution of the screen, the graphics would get deformed in such ways that it'd get almost unbearable to see, which would make the gameplay more confusing than it should be.
Also, controls are the biggest issue.
If anyone wants to mention the Wiimote as proof that D-Pads work for Smash Bros, I'll immediately just point towards the Wiimote as proof that D-Pads will never work for Smash Bros. The distinction between tilting tapping the stick is
very important to Smash's basic gameplay in a way that double-pressing a direction will just never do the work properly.
Hey, how's this? Grab a Wiimote, and try to do an uptilt without having to do any actions frist. Aaaaaaaaand you can't, you have to either Smash, or jump. Or how about an ftilt? Can you do one straight from a standing position without having to move forward a little bit? Nay. It just doesn't work well without an analog stick.
So would it be technically possible? Definitely, and I say they wouldn't even need to do such a low number of characters, they could perfectly have a good number, after all, the GBA has this thing:
The problem is that I think it simply wouldn't have played too well, even if it's technically possible for the system.