Guys he's clearly not dissing 50 cent either.
What he's saying is that because Em fitted in, he inspires black rappers to try and mimic him. That line isn't about any rapper, he's just speaking in general terms. That is, the way Eminem looked up to black rappers when he was starting up, Lupe wished of a world where at the time Eminem came up, black rappers could look at him and try and mimic him. The 'mimic' is not said in a negative manner either.
Then he mentions 50 Cent in that maybe in a different world a white rapper like 50 Cent existed and was signed by Eminem. This little line here can be taken in many ways - where he's referring to the type of life 50 Cent came from, the idea that Eminem could sign a white rapper like that and that not just black rappers like 50 Cent existed. It's mostly a shot at the system, nothing but style and grace when mentioning 50's name though. In fact, its a line that makes you think a lot, and makes you wonder what if a white rapper like 50 existed.