Masked Man technically is not a villain, since he is a puppet.
If they were gonna add a villain from Mother, I think Porky would be the logical choice:

Which… may be worse for Lucas, knowing that he was the one that "created" the masked man and the one to blame for what happen to Lucas's Mom.
Also, it was a villain for Ness too.

Porky is one of the characters I actually want to be in Smash. Also Gigue or Earthbound 1 Giygas as an echo fighter of Mewtwo. But I would like to add that he should probably appear in his Mother 3 mech and before you start saying it's too big, for whatever reason Sakurai made it huge for Brawl for some reason.

Like in brawl it's like 3 porky's tall and the bed part is 3 porky's wide while in Mother 3 it's it's about 2 porky's wide and I don't even think 2 porky's tall.