That actually was the "story / rumor" around that time. Eddie was doing well going into that award season. He won the SAG Award, Golden Globe, Critics Choice, & was nominated.

Norbit being released in January (and the response to it), was widely circulated as being the reason he lost. Eddie was a (likable) Hollywood star for 3 decades...this was his first nomination, so I got the feeling that goodwill, along with a fairly strong performance in Dreamgirls, would get him the win. The Norbit came out.

I'm not if Norbit deserves 100% of the blame, but I definitely think it did not help his case.