Right now, I'm thinking it's legit.
Things to note:
-Throughout the development of Smash Bros., characters tended to be confirmed as part of the roster to coincide with the release of an associated game. Lucas coming back to Smash while Mother 3 releases for the first time in most of the world would be no coincidence if this were true.
-Nintendo released Earthbound for the first time in Europe in 2013 through the Virtual Console, indicating that Nintendo is aware of the interest in the series.
-When Smash Bros. was released, the European version listed Earthbound's release date as the day it appeared on Virtual Console, as it was the first time it appeared in that region. If Mother 3 were to be released worldwide, and it gets added to Smash Bros. as part of the Lucas DLC, that date would indeed be consistent. Except in Japan.
-As Mother 3 would be a Virtual Console release, I don't think it's too sudden an announcement. N64 games came to Virtual Console the day they were announced. I would expect the next Nintendo Direct, or possibly even sometime during E3 for an official announcement regarding Mother 3.
If it's fake, it's a very impressive fake. The guy did his homework.