I’m pretty sure Leonard Cohen wasn’t really your average teenager’s jam at the time Shrek came out. A lot of us knew the song, but I’m pretty sure a lot of us wouldn’t be able to tell you who sang it. Smash Mouth didn’t really get much airtime over here either before Shrek, as far as I can remember, and the movie was amongst the top grossing movies of the year IIRC, so it’s not that surprising that many people discovered them both through the movie.