Zoma would be super cool as a playable character or even just as a boss. However, I think Dragon Quest in general has the "Fire Emblem" problem regarding that: just like every hero is different, so is every villain, so it's tough knowing who to choose. Zoma is also a twist villain, in that we learn he's been behind Baramos' actions the whole time, and we don't learn too much about Zoma either.
Smash currently doesn't have any one-off villains playable. Bowser, Bowser Jr., the Koopalings, Piranha Plant, King K. Rool, Ganondorf, Ridley, Dark Samus, Meta Knight (?), King Dedede (?), and Wolf are all recurring characters in their series. The closest thing we have to a one-off villain is Dark Pit, but he's not really a villain and he was an extremely easy Echo Fighter to add.
The merit I see with Zoma is that, like Erdrick, he's part of the most important Dragon Quest game, so he could slip in that way. I would personally have a villain the game spends the entire time building up / getting you familiar with, like the Dragonlord or Psaro the Manslayer, maybe even Bishop Ladja, but Zoma could still be cool.