Inb4 Smash 5/6 comes out and we say Smash 4's visuals haven't aged well.
But yes, I noticed that Melee's visuals haven't aged well. Same with Brawl.
I actually don't think there's much to improve on this games' visuals. I think the graphical evolution of games overall is more akin to a logarithmic rate. Very fast improvements at the beginning, but those will grow slower and slower as the time passes.
I think Brawl's style made a great first impression at the time, but aged horribly.
Melee hasn't aged as much as Brawl in regards to art style, but it really l looks aged in regards to animation work. The animations are less fluid, something especially notable when characters charge their Smash attacks. The victory animations aren't really great either.
Brawl's animation work, for example, has aged well.
Smash 4 has a special charm that I think only Smash 64 captures as well. The characters look a lot more like they do in their own games. I feel it's not something Melee and Brawl capture as well as 64 and Wii U/3DS do.
For example, despite still being a Captain Falcon semi-clone (which at this point is a Grandfather Clause stemming from his last-minute addition in Melee) and how I would like him to be further differentiated (though I disagree with the notion of a complete overhaul, but rather something along the lines of what Mario, Luigi, Falco, Bowser and Pit had throughout the series), I really appreciate the animation work put into him to make him feel as more Ganon-like as possible within the constraints he has a Falcon clone. This is why I can't like seeing him in Melee anymore, where most of his animations are copy-pasted from Falcon.