With a lottery OCI, technically yes, a poor student could get an interview with a top firm, and the top student might not get one.
So where do grades, etc. come into play? On the callbacks. Anyone can get their foot in the door for that initial interview...the school's trying to make it an even playing field (hoping maybe a poor student can impress with non-grade attributes, so that the firm may not even care once it sees the students grades).
But after that initial interview, that's when recruiters will take a look at your transcript. And likely offer callbacks only to those who interviewed well AND have the "appropriate" grades, etc.
Having said that, if you aren't selected in the lottery for a firm you really want to interview with, then talk to your counselor about options...whether just signing some kinda waitlist on the interview and camping outside the door or mailing your info to that firm outside of OCI.