That depends on whether you think true fighters are born or made. I believe fighters are made and defined by the community, so Smash (particularly Melee) is a legit fighter imo. You could even make a single Mario Party minigame competitive if it had a large enough community and deep enough gameplay.
I'm talking about competitive fighting games, not making other games competitive in general. Also, Mario Party is a terrible example. Its like saying the mini games in Barbies Pony Adventures and Pokemon Stadium 2 could be competitive games if the fan base was large enough. Come on, really? Fighting games are made.
Smash was changed using random exploits in its physics engine, and smarter methods of play. People found fun in learning these new techniques and challenging others. None of this makes the game a fighter. It just has a lot of similarities to one because of all the improvements the game's community made on it.
Smash is a fighter, the only true difference between smash and other fights is an increase focus on zoning. Zoning takes a much smaller place in other fighting games, but in Smash it is essential, to some characters more than others such as Falco, the Links, and Snake. Where you are on Jungle Japes is going to drastically alter how the game carries on from there, based on both players ability to handle the stage correctly.
Yomi still takes place in Smash in the exact same way as it does in other fighters.
Smash was made by its creators to be a game you could play with a ****load of people and it serves its purpose well. I've had more fun with this game than any others I've ever played.
As a fighter, I think it also works. I will say, though, that it is one of the most unconventional fighters ever made. An increased focus on zoning being the only true difference? Are you kidding me?
First, because of how limited movement is in traditional fighters, zoning is SO much more important in those games. The 2D fighter community is all about that kind of spacing. There are any platforms to help with approaches. There is only jump in, walk forward, or tip your moves for safe pokes. Zoning is alot more important in SF/GG than you think it is. Soul Calibur could be an exception though, with its 8-direction movement system. 3D fighters are a different story entirely.
There is NO other competitive game that has a movement system as open-ended as smash. Learning how to work platforms for approach/defense and 'mindgamey' purposes. Wavedashing and its multiple variants for combo extentions, safe edge returns, spacing, and just plain flash. Movement is one huge aspect of this game that sets it apart from the rest.
Secondly:
Smash took things to a whole level by taking away the health bar and implementing the percentage aspect. This indirectly lead to a VERY important technique we've come to know as
edge-guarding. It was a completely new and creative concept that helped propel the game competitively. It gave the game more depth when it came to move filtering and decision-making. Make the wrong choice and pick the wrong move, you fail and they come back. They then use this failure to their advantage. The push-pull scenarios were also augmented through the use of movement and edge-guarding; creating the illusion of lag to force your opponent to commit to a move then punishing.
I will admit that the game got more SF like as it developed...with high damage combos off spaced hits, zoning wars, mixups and the like. In my opinion, it still isn't a true fighter. It still is my favourite game of all time though.