I don't know. Sonic would probably be a cool SSB character, but Sonic games are for all systems. Sega isn't teamed up with any company, so if they put in Sonic, they might as well put in other third party characters, like characters from Soul Calibur, Mortal Kombat, Turok, Metal Gear, Rayman, etc.
But if Nintendo and Sega were teamed up, and Sonic was a Nintendo only thing, I'd probably say yes. That wouldn't be much different than putting characters like Geno and Mallow, who weren't made by Nintendo, but are Nintendo specific. Well, it would still be different because Sonic is much more a Sega thing, even if Sega started only making Sonic games for Nintendo.
But on the other hand, Sonic could make a great Smash character. Maybe he could be with Tails, like the Ice Climbers. And then, like, Tails would be all...flying. And stuff. Err, now that I think about it, it would probably be pretty wierd if Tails was like that.