Masked Man isn't really necessary in the equation for Mother series representation. Although he's important to Mother 3 his appearances are rather sporadic, and I don't think he's as much a face of the game as Porky or the protagonists.
I think Ness as the protagonist of Earthbound, Lucas as the protagonist of Mother 3, and a Mother 1 stage (and maybe a Ninten alt for Ness) would cover all the bases perfectly. It is a small franchise, and while I love Mother the last thing I'd want is to see it obscenely overrepresented like Kid Icarus.