I have somewhat heard of it before playing Melee. I read something about three of the secret characters in Melee itself in an article in Nintendo Power magazine, and Marth was among those three SSBM characters. (The other two being Ganon and Luigi.) And when I read his description in Nintendo Power magazine, I had then known FE was an RPG, but I didn't know how it was. (And yet, when I did get and play Melee, as a guy who didn't even play FE at the time, I knew right away that Marth and Roy weren't from the same exact game. That was a common misconception at the time. All thanks to carefully reading, not only their trophy descriptions in english, but also what platform their games were on via the Japanese language option.)
EDIT: I had no interest in it even after SSBM, but after I picked up FE9, I've became very interested in it.