Except that Mother 3 hasn't been released outside of Japan. Yes, I know Melee was released with Fire Emblem characters in it before any Fire Emblem games were released outside of Japan, but Mother 3 has no chance of being released while Fire Emblem did.
I say this not based on Mother 3's popularity, but based on the fact that Sakurai has said in the past that he had wanted to include Lucas rather than Ness in the first place, but that Earthbound 64 kept getting pushed back so he had to put in Ness instead, and because it's been stated elsewhere that a track from Mother 3 was being included in SSBB.
Now admittedly, that Mother 3 track could just be alternate music for Ness's stage or something, but I'm really not going to be surprised if Lucas replaces him.
Also, I wouldn't say Mother 3 has
no chance of being released. Just
practically no chance. NOA has at least admitted to being aware of it and being aware of fan interest and the sales records in Japan, at least.