Meh, I like/sometimes love Cheryl's style, but I don't think she's a fashion icon. To me, iconic is something you look back on years from now and think, "Wow, she had really good style." You can't really say Cheryl is a fashion icon because she's only really been relevant in the fashion community for the past 2 - 3 years.
Contrast that to Jackie O or Audrey Hepburn, whose styles were so distinct and are still remembered today. Princess Diana might fit in that category too. Marilyn Monroe's white dress is iconic because we ALL remember that image.
The thing is, we're all talking about Cheryl's fashion a lot because it's happening right now. We have new outfits to discuss and everything. But 10, 20 years from now, will we still be pulling up these old images and saying, "Wow, I really liked her style!" Will she still have many people talking about her outfits? To me, that's the mark of a true icon, versus someone who is just a passing fad/phase.
But everyone's definition of icon is different.
As for Cheryl... at the moment, I don't really see her achieving iconic status. First, like previously mentioned, she hasn't set any trends. Second, her style itself doesn't stand out. She looks gorgeous and great, but I don't see any of her outfits as distinctly "Cheryl", whereas I could see it with Gwen Stefani or Chloe Sveginy maybe.