I wouldn't say the Duchess's style is "boring" or "bland", but she has long proven herself to be something of a traditionalist when it comes to clothes, and there's nothing wrong with that. The coat/dress combinations that she likes are very "royal" in their approach, and perhaps for the time being, that's what she feels most confident wearing as she assumes her new role. But it's also possible that they are simply what she LIKES best, and the fact is that she wears them quite well. On other women, older and with other physiques, they can look dowdy, but they do not on her.
Yes, she may become more adventurous -- the McQueen coat for the Trooping event was fairly progressive in style -- but very high style, very progressive clothing is often very, very expensive (and not easily recyclable), and my guess is that she's going to continue to wear a lot of lower-end, versatile pieces in which she feels comfortable and safe.
As for being a style icon, it isn't necessary to be all that unique and personal in one's choices to become one; you simply need to make admired and influential choices. Jackie Kennedy is a good example of an icon whose clothes were beautiful and elegant but not particularly unusual for her day. A style icon is an icon because people not only admire her style, they feel they can emulate it. That's hard to do if a woman's style is so unique and personal that it would work (or be affordable) for very few other people.