I wouldn't say that, he wasn't that big of a deal before Eddie died
Yeah, he was. He was glorifed immensely during his very first WCW matches, and he's a G-d in Mexico according to Alberto Del Rio. He's one of the greatest technical wrestlers of all time AND one of the greatest traditional luchadores, AND he's colorful and different, and the product always needs different stars so everyone has something to enjoy (and so individuals get different things out of one card).
Seriously, Rey's a legend. The fact that it took as long as it did for him to win the belt is what's surprising. Its more likely that happened after Eddie's death because that signaled the end of the SD golden age. Lesnar gone, Guerrero gone, all they had left was Angle and Taker (not knocking them). So they needed to create stars, and Mania's the time to do it.
And so what if he was a token Mexican? Mark Henry's a token giant, Ziggler's a token pretty boy (and I think one of the greatest of his generation). And if he got inserted into the position cuz Eddie died, then apparently Eddie was the token Mexican then too. John Cena's the token role model, Batista/Sheamus is the token best friend, Sheamus is kindof the token Brit (I know he's not British, but he serves the role on some level). If Benoit gets a title reign despite no microphone skills, based solely on in-ring talent, then Mysterio is just as qualified. Jack Swagger g-d damn it.