Yeah I used to not like Natalie when she came into the picture, because she replaced Sharona and I was used to her. But I adjusted, and now that I look at it, Natalie seems better fit. They both get angry with him and disagree with Mr.Monk at times, but Natalie seems more stable in her decision to stay by his side. She's only quit him once, right? in that lottery episode. I remembered Sharona did it on more than one occasion.
Natalie seems to understand him more, and her devotion to Monk is much greater. That's especially proven in the episode Monk was framed for murder and eventually faked his death, etc etc etc.
Monk and Natalie look better standing side by side, anyways. XD They're clothing were casual, and reserved. Something about their facial features match, also. Throw Sharona next to Monk and she sticks out, what not with her curls, hoop earrings, and skirts.
But Sharona DID help Mr.Monk get back on his feet after the Trudy incident, so give her props for that at least.