I've heard of her and I like her music. First of all, I have to say that there is no such thing as "reverse racism". Discriminating against a person based on the color of their skin (regardless of whether they are black, white, whatever) is racism. Period.
Your theory is not correct. Lady Sovereign (LS) herself has said that she has gotten more attention because she is white and British in a music category with predominantly black, American artists. She has done extremely well for someone who had no ties (i.e. relatives) to the music industry before she got started. Some accomplishments since she signed on with Jay-Z's company:
*" Love Me or Hate Me" went to #1 on MTV's TRL - the first time a British artist has been #1,
*"Love Me or Hate Me" went to #1 on the Billboard dance chart,
*She has never been on Jay Leno but she has appeared on Letterman, Jimmy Kimmel, and Craig Ferguson's late night talk shows,
*She performed during the 2007 MTV New Year show and was a presenter at the MTV Woodie Awards,
*LS was the opening act for the U.S. leg of Gwen Stefani's Sweet Escape tour.
I think that she hasn't done better because she doesn't work as hard as some other artists. Rihanna signed with Def Jam around the same time that LS did. Since then Rihanna has recorded 3 albums while LS has only put out 1. And most of the songs on her Public Warning album were recorded before she signed with Jay-Z's company.
It is a logistical problem that many of the songs - 9 to 5, Random, Public Warning, Blah Blah, A Little Bit Of Shhh, Hoodie, Tango, and Fiddle With the Volume - were recorded before LS signed with Def Jam. That means that Def Jam can't (or won't) release them as singles because LS's old record company would get most of the profit from those sales. No single means no record company $$ for promotion which means no new video which means no radio airplay.
LS has a reputation for not playing very well with others. She has publicly criticized many other singers who have sold more albums than she has - Jessica Simpson, Fergie, Lily Allen, Corinne Bailey Rae, Nelly, 50 Cent - and said that all the albums that came out during 2006 were sh!t. LS had a recording session with one of the greatest producers in the industry - Pharrell - and she couldn't come up with even one new song. LS does not enjoy doing interviews with the press which can make it difficult for her to get good publicity. During her radio interviews she sounds bored and thus comes across as boring and uninteresting.
LS is at her best when she's performing live. Unfortunately she has also had some very public onstage freakouts that did nothing to bolster her reputation.
LS suffered a bout of depression that sidelined her for awhile at the beginning of the year. Hopefully she's better now. She's been working on a new album since she finished touring with Gwen. If it turns out well she could have a reinvigorated career. If it doesn't she could be written off as a one hit wonder who was once promising but couldn't deliver.