I disagree with this. While I don't deny that his popularity mostly comes from being in Smash, I very much doubt being an "original" to Smash character (which technically he isn't) is what caused his popularity. If there was an original to Smash character (such as primid, or an alloy, or something new, etc) that was playable, I would guess opinions of it and its popularity would be very divided, and it would not gain enough undisputed popularity to be one of the most popular characters in the game, such as Roy was.
Even if the character was created for another series, but had their debut in Smash, (which isn't a true original character) like Roy, and got their popularity in that regard, logically the popularity would dissipate after the game had been out for a time. This didn't happen to Roy, his popularity remained strong after his game came out. Plus the vast majority of non-Japanese people who played Melee had no idea that Roy's game wasn't out yet.
Like Chrono said, most of his popularity with the average Smasher came from his cool appearance and attacks, and his relative ease of use.